search/search-engine: Replace "context" property with context getter method

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/702>

src/core/gcal-context.c

@@ -164,7 +164,6 @@ gcal_context_constructed (GObject *object)
   G_OBJECT_CLASS (gcal_context_parent_class)->constructed (object);
 
   self->manager = gcal_manager_new ();
-  self->search_engine = gcal_search_engine_new (self);
 }
 
 static void
@@ -466,4 +465,7 @@ gcal_context_startup (GcalContext *self)
     }
 
   gcal_manager_startup (self->manager);
+
+  g_assert_nonnull (g_application_get_default ());
+  self->search_engine = gcal_search_engine_new ();
 }