window: Use AdwButtonContent for Today button

Text buttons in headerbars should come with an icon,
or an arrow if they're a dropdown. For this, we
have AdwButtonContent.

This commit makes use of AdwButtonContent for
the Today button, adding an icon and allowing
for the button to be flat in the headerbar.

src/gui/gcal-window.ui

@@ -121,8 +121,13 @@
             </child>
             <child>
               <object class="GtkButton" id="today_button">
-                <property name="label" translatable="yes">Today</property>
                 <property name="action-name">win.today</property>
+                <property name="child">
+                  <object class="AdwButtonContent">
+                    <property name="label" translatable="yes">Today</property>
+                    <property name="icon-name">calendar-today-symbolic</property>
+                  </object>
+                </property>
               </object>
             </child>
             <child>

src/gui/icons/calendar-today-symbolic.svg

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" height="16px" viewBox="0 0 16 16" width="16px"><g fill="#2e3436"><path d="m 3.042969 1 c -1.128907 0 -2.042969 0.914062 -2.042969 2.042969 v 9.875 c 0 1.132812 0.914062 2.042969 2.042969 2.042969 h 9.914062 c 1.128907 0 2.042969 -0.910157 2.042969 -2.042969 v -9.875 c 0 -1.128907 -0.914062 -2.042969 -2.042969 -2.042969 z m -0.042969 4.960938 h 10 v 7 h -10 z m 0 0"/><path d="m 5 7 v 4 h 4 v -4 z m 1 1 h 2 v 2 h -2 z m 0 0"/></g></svg>

src/gui/icons/icons.gresource.xml

@@ -4,5 +4,6 @@
     <file compressed="true">calendar-month-symbolic.svg</file>
     <file compressed="true">calendar-week-symbolic.svg</file>
     <file compressed="true">calendar-year-symbolic.svg</file>
+    <file compressed="true">calendar-today-symbolic.svg</file>
   </gresource>
 </gresources>