label: Deny drag gesture when not selectable

We don't actually do anything when the label is not selectable
except for consuming the event, which breaks for instance titlebar
drags with labels that contain links. Simply deny the gesture in
that case to allow the event to bubble up normally.

https://bugzilla.gnome.org/show_bug.cgi?id=759798

gtk/gtklabel.c

@@ -5147,7 +5147,7 @@ gtk_label_drag_gesture_begin (GtkGestureDrag *gesture,
   const GdkEvent *event;
   gint min, max, index;
 
-  if (!info)
+  if (!info || !info->selectable)
     {
       gtk_gesture_set_state (GTK_GESTURE (gesture), GTK_EVENT_SEQUENCE_DENIED);
       return;