js: Name all the timeouts and idles

With very uninventive names. Names now, good names later.

https://bugzilla.gnome.org/show_bug.cgi?id=727983

js/ui/dnd.js

@@ -395,6 +395,7 @@ const _Draggable = new Lang.Class({
 
         this._updateHoverId = GLib.idle_add(GLib.PRIORITY_DEFAULT,
                                             Lang.bind(this, this._updateDragHover));
+        GLib.Source.set_name_by_id(this._updateHoverId, '[gnome-shell] this._updateDragHover');
     },
 
     _updateDragPosition : function (event) {