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/meson.build

@@ -17,6 +17,7 @@ built_sources += gnome.compile_resources(
 sources += files(
   'gcal-application.c',
   'gcal-calendar-list.c',
+  'gcal-calendar-navigation-button.c',
   'gcal-drop-overlay.c',
   'gcal-event-popover.c',
   'gcal-event-widget.c',