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/views/gcal-week-view.c

@@ -570,6 +570,7 @@ gcal_week_view_set_property (GObject       *object,
       break;
 
     case PROP_CONTEXT:
+      g_assert (self->context == NULL);
       self->context = g_value_dup_object (value);
 
       gcal_week_grid_set_context (GCAL_WEEK_GRID (self->week_grid), self->context);
@@ -580,8 +581,6 @@ gcal_week_view_set_property (GObject       *object,
                                G_CALLBACK (gtk_widget_queue_draw),
                                self->hours_bar,
                                G_CONNECT_SWAPPED);
-
-      g_object_notify (object, "context");
       break;
 
     default: