client: Running as a service does not mean running in the background indefinitely

This is wrong as Geary is mostly started via D-Bus activation: it will never quit.

Ignore --hidden option, it will be removed soon.

Fix #728

src/client/application/application-client.vala

@@ -210,10 +210,7 @@ public class Application.Client : Gtk.Application {
      */
     public bool is_background_service {
         get {
-            return (
-                (this.flags & ApplicationFlags.IS_SERVICE) != 0 ||
-                this.start_hidden
-            );
+            return this.config.startup_notifications;
         }
     }