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/gcal-edit-dialog.c

@@ -1202,6 +1202,7 @@ gcal_edit_dialog_set_property (GObject      *object,
       break;
 
     case PROP_CONTEXT:
+      g_assert (self->context == NULL);
       self->context = g_value_dup_object (value);
       g_signal_connect_object (self->context,
                                "notify::time-format",