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-month-view.c

@@ -2182,6 +2182,7 @@ gcal_month_view_set_property (GObject       *object,
       break;
 
     case PROP_CONTEXT:
+      g_assert (self->context == NULL);
       self->context = g_value_dup_object (value);
 
       for (i = 0; i < 42; i++)
@@ -2199,8 +2200,6 @@ gcal_month_view_set_property (GObject       *object,
                                self,
                                0);
       update_weather (self, TRUE);
-
-      g_object_notify (object, "context");
       break;
 
     default: