Move sync indicator into a separate widget

And move the new widget into the headerbar itself, instead of
the calendars button.

src/gui/meson.build

@@ -23,6 +23,7 @@ sources += files(
   'gcal-overflow-bin-layout.c',
   'gcal-quick-add-popover.c',
   'gcal-search-button.c',
+  'gcal-sync-indicator.c',
   'gcal-toolbar-end.c',
   'gcal-weather-settings.c',
   'gcal-window.c',