gui: introduce navigation button with date chooser

The navigation button provides access to a date chooser
at all times without requiring it to be permanently
visible, as done currently with the always-shown sidebar.

Signed-off-by: Markus Göllnitz <camelcasenick@bewares.it>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/542>

src/gui/gui.gresource.xml

@@ -2,6 +2,7 @@
 <gresources>
   <gresource prefix="/org/gnome/calendar/ui/gui">
     <file compressed="true">gcal-calendar-list.ui</file>
+    <file compressed="true">gcal-calendar-navigation-button.ui</file>
     <file compressed="true">gcal-event-popover.ui</file>
     <file compressed="true">gcal-event-widget.ui</file>
     <file compressed="true">gcal-meeting-row.ui</file>