Add gtk_widget_prepend_controller

Add a variant of gtk_widget_add_controller that
inserts the controller at the beginning, instead
of the end. This will be used in entry completion
to make sure the entry completion key event handling
supersedes the entry one while the popup is open.

Keep this private for now, until we determine if
it needs to be public api.

gtk/gtkwidget.c

@@ -11415,6 +11415,23 @@ gtk_widget_add_controller (GtkWidget          *widget,
     gtk_list_list_model_item_added_at (priv->controller_observer, priv->controllers->len - 1);
 }
 
+void
+gtk_widget_prepend_controller (GtkWidget          *widget,
+                               GtkEventController *controller)
+{
+  GtkWidgetPrivate *priv = gtk_widget_get_instance_private (widget);
+
+  GTK_EVENT_CONTROLLER_GET_CLASS (controller)->set_widget (controller, widget);
+
+  if (G_UNLIKELY (!priv->controllers))
+    priv->controllers = g_ptr_array_new ();
+
+  g_ptr_array_insert (priv->controllers, 0, controller);
+
+  if (priv->controller_observer)
+    gtk_list_list_model_item_added_at (priv->controller_observer, 0);
+}
+
 /**
  * gtk_widget_remove_controller:
  * @widget: a #GtkWidget

gtk/gtkwidgetprivate.h

@@ -346,6 +346,9 @@ gboolean          gtk_widget_run_controllers               (GtkWidget
                                                             double               x,
                                                             double               y,
                                                             GtkPropagationPhase  phase);
+
+void              gtk_widget_prepend_controller            (GtkWidget             *widget,
+                                                            GtkEventController    *controller);
 void              gtk_widget_handle_crossing               (GtkWidget             *widget,
                                                             const GtkCrossingData *crossing,
                                                             double                 x,