gui: Move CalendarComboRow to common

CalendarComboRow is used both by the event editor and the importer.
Moving it to common for a better file organization.

src/gui/meson.build

@@ -1,4 +1,5 @@
 subdir('calendar-management')
+subdir('common')
 subdir('event-editor')
 subdir('gtk')
 subdir('icons')
@@ -16,8 +17,6 @@ built_sources += gnome.compile_resources(
 sources += files(
   'gcal-application.c',
   'gcal-calendar-button.c',
-  'gcal-calendar-combo-row.c',
-  'gcal-calendar-combo-row-item.c',
   'gcal-drop-overlay.c',
   'gcal-event-popover.c',
   'gcal-event-widget.c',