Fix GcalContext management

GObject automatically notifies when passing
properties as part of g_object_new(). This
happens after the instance is created, thus
after constructed().

By passing G_PARAM_EXPLICIT_NOTIFY, we avoid
this behavior. However, we are still notifying
PROP_CONTEXT in GcalWindow. Removing this other
notify fixes the crashes related to GcalContext.

https://gitlab.gnome.org/GNOME/gnome-calendar/issues/417

src/gui/calendar-management/gcal-calendar-management-dialog.c

@@ -247,7 +247,6 @@ gcal_calendar_management_dialog_set_property (GObject      *object,
       g_assert (self->context == NULL);
       self->context = g_value_dup_object (value);
       setup_context (self);
-      g_object_notify_by_pspec (object, properties[PROP_CONTEXT]);
       break;
 
     default: