application: handle ref count of GtkCssProvider correctly

It seems gtk_style_context_add_provider_for_screen increase the ref
count of the GtkCssProvider (maybe copy it to GdkScreen and this isn't
releasing it correctly). We're forcing an _unref to keep the destruction
process working as expected. Previously gtk_css_provider_finalize was
not called at any time on the closing process.

src/gcal-application.c

@@ -187,6 +187,7 @@ gcal_application_startup (GApplication *app)
                    NULL);
 
      g_object_unref (css_file);
+     g_object_unref (priv->provider);
    }
 
   priv->manager = gcal_manager_new ();