general: Use GcalToolbarEnd for toolbars

For the sake of smooth adaptivity, we need the toolbar buttons
to be constant in both modes instead of moving places from one
mode to another. Instead of duplicating these groups of
buttons, we can refactor them into an object for toolbars
and use them in multiple places.

This commit takes the widgets from the end of the header bar and
creates `GcalToolbarEnd`, which is used in the header bar and the
mobile action bar.

src/gui/gui.gresource.xml

@@ -8,6 +8,7 @@
     <file compressed="true">gcal-quick-add-popover.ui</file>
     <file compressed="true">gcal-search-button.ui</file>
     <file compressed="true">gcal-search-hit-row.ui</file>
+    <file compressed="true">gcal-toolbar-end.ui</file>
     <file compressed="true">gcal-weather-settings.ui</file>
     <file compressed="true">gcal-window.ui</file>
   </gresource>