gui/event-editor: Add time zone selection

Creates a chooser managing a GDateTime. The chooser allows the user
to change the date, the time and the timezone independently.

This replaces the precedent GcalTimeChooserRow widget, because it is
easier to manage the different interactions with the user in a single
widget.

Co-authored-by: Georges Basile Stavracas Neto
<georges.stavracas@gmail.com>

src/core/gcal-event.c

@@ -234,7 +234,7 @@ build_component_from_datetime (GcalEvent *self,
       g_autoptr (GTimeZone) zone = NULL;
       ICalTimezone *tz;
 
-      zone = gcal_util_get_app_timezone_or_local ();
+      zone = g_date_time_get_timezone (dt);
       tz = gcal_timezone_to_icaltimezone (zone);
       i_cal_time_set_timezone (itt, tz);
       tzid = g_strdup (i_cal_timezone_get_tzid (tz));