Don't assert in gtk_event_controller_constructed

This prevents some of our generic object implementation tests
from working with gesture objects. Instead, add g_return_if_fail
checks in all the gesture constructors.

gtk/gtkgesturedrag.c

@@ -186,6 +186,8 @@ gtk_gesture_drag_init (GtkGestureDrag *gesture)
 GtkGesture *
 gtk_gesture_drag_new (GtkWidget *widget)
 {
+  g_return_val_if_fail (GTK_IS_WIDGET (widget), NULL);
+
   return g_object_new (GTK_TYPE_GESTURE_DRAG,
                        "widget", widget,
                        NULL);