icon: avoid using text in app icon

- new icon metaphor to avoid using letters/numbers

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/issues/351

data/icons/hicolor/symbolic/apps/org.gnome.Calendar-symbolic.svg

@@ -1,58 +1 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<svg
-   xmlns:dc="http://purl.org/dc/elements/1.1/"
-   xmlns:cc="http://creativecommons.org/ns#"
-   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-   xmlns:svg="http://www.w3.org/2000/svg"
-   xmlns="http://www.w3.org/2000/svg"
-   width="16.000061"
-   version="1.1"
-   style="enable-background:new"
-   id="svg7384"
-   height="16">
-  <metadata
-     id="metadata90">
-    <rdf:RDF>
-      <cc:Work
-         rdf:about="">
-        <dc:format>image/svg+xml</dc:format>
-        <dc:type
-           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
-        <dc:title>Gnome Symbolic Icons</dc:title>
-        <cc:license
-           rdf:resource="http://creativecommons.org/licenses/by-sa/4.0/" />
-      </cc:Work>
-      <cc:License
-         rdf:about="http://creativecommons.org/licenses/by-sa/4.0/">
-        <cc:permits
-           rdf:resource="http://creativecommons.org/ns#Reproduction" />
-        <cc:permits
-           rdf:resource="http://creativecommons.org/ns#Distribution" />
-        <cc:requires
-           rdf:resource="http://creativecommons.org/ns#Notice" />
-        <cc:requires
-           rdf:resource="http://creativecommons.org/ns#Attribution" />
-        <cc:permits
-           rdf:resource="http://creativecommons.org/ns#DerivativeWorks" />
-        <cc:requires
-           rdf:resource="http://creativecommons.org/ns#ShareAlike" />
-      </cc:License>
-    </rdf:RDF>
-  </metadata>
-  <title
-     id="title8473">Gnome Symbolic Icons</title>
-  <defs
-     id="defs7386" />
-  <g
-     transform="translate(-112,-668)"
-     id="layer3">
-    <path
-       d="M 115.71484,669 C 114.22843,669 113,670.22842 113,671.71484 v 8.57032 c 0,1.48641 1.22843,2.71484 2.71484,2.71484 h 8.57032 C 125.77157,683 127,681.77157 127,680.28516 v -8.57032 C 127,670.22842 125.77157,669 124.28516,669 Z m 0,2 h 8.57032 c 0.41301,0 0.71484,0.30182 0.71484,0.71484 v 3.6211 L 124.33594,676 125,676.66406 v 3.6211 c 0,0.413 -0.30183,0.71484 -0.71484,0.71484 h -8.57032 C 115.30183,681 115,680.69816 115,680.28516 v -3.57813 L 115.70703,676 115,675.29297 v -3.57813 C 115,671.30182 115.30183,671 115.71484,671 Z"
-       id="path864"
-       style="color:#000000;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:medium;line-height:normal;font-family:sans-serif;font-variant-ligatures:normal;font-variant-position:normal;font-variant-caps:normal;font-variant-numeric:normal;font-variant-alternates:normal;font-feature-settings:normal;text-indent:0;text-align:start;text-decoration:none;text-decoration-line:none;text-decoration-style:solid;text-decoration-color:#000000;letter-spacing:normal;word-spacing:normal;text-transform:none;writing-mode:lr-tb;direction:ltr;text-orientation:mixed;dominant-baseline:auto;baseline-shift:baseline;text-anchor:start;white-space:normal;shape-padding:0;clip-rule:nonzero;display:inline;overflow:visible;visibility:visible;opacity:1;isolation:auto;mix-blend-mode:normal;color-interpolation:sRGB;color-interpolation-filters:linearRGB;solid-color:#000000;solid-opacity:1;vector-effect:none;fill:#000000;fill-opacity:1;fill-rule:nonzero;stroke:none;stroke-width:2;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-dashoffset:0;stroke-opacity:1;color-rendering:auto;image-rendering:auto;shape-rendering:auto;text-rendering:auto;enable-background:accumulate" />
-    <path
-       d="M 116.51791,673 116,674.03584 h 1.01396 V 679 H 119 v -6 z m 3.43732,0 v 3.21297 c 0,1.03581 0.77686,1.03581 1.29477,1.03581 h 0.75583 V 679 H 124 V 676.10232 673 h -1.99417 v 3 h -0.72458 c -0.25896,0 -0.25896,-0.25892 -0.25896,-0.25892 V 673 Z"
-       id="path1058"
-       style="font-style:normal;font-variant:normal;font-weight:bold;font-stretch:normal;font-size:medium;line-height:1.25;font-family:Montserrat;-inkscape-font-specification:'Montserrat, Bold';font-variant-ligatures:normal;font-variant-caps:normal;font-variant-numeric:normal;font-feature-settings:normal;text-align:start;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:start;display:inline;fill:#241f31;fill-opacity:1;stroke:none;stroke-width:0.24999994;enable-background:new" />
-  </g>
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="16" height="16"><defs><filter id="a" filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%"><feColorMatrix in="SourceGraphic" values="0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 1 0"/></filter><mask id="d"><g filter="url(#a)"><path fill-opacity=".542" d="M0 0h16v16H0z"/></g></mask><clipPath id="b"><path d="M0 0h192v152H0z"/></clipPath><g id="c" clip-path="url(#b)" fill="#c0bfbc"><path d="M28 120c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M44 120c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M60 120c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M28 97c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M44 97c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M60 97c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M76 97c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M92 97c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M108 97c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M44 74c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M60 74c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M76 74c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M92 74c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0M108 74c0 2.21-1.79 4-4 4s-4-1.79-4-4 1.79-4 4-4 4 1.79 4 4zm0 0"/></g></defs><use xlink:href="#c" transform="translate(-160 -24)" mask="url(#d)"/><path d="M4 10h2v2H4zm6 0h2v2h-2zm-3 0h2v2H7zm3-3h2v2h-2zM7 7h2v2H7zM3.715 1A2.73 2.73 0 0 0 1 3.715v8.57A2.73 2.73 0 0 0 3.715 15h8.57A2.73 2.73 0 0 0 15 12.285v-8.57A2.73 2.73 0 0 0 12.285 1zM3 6h10v6.285c0 .414-.3.715-.715.715h-8.57A.692.692 0 0 1 3 12.285zm0 0" fill="#241f31"/></svg>