js: Use SOURCE_CONTINUE/SOURCE_REMOVE constants in source functions

With support for boolean constants in g-i, we can finally use the
more readable constants instead of true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567

js/ui/lookingGlass.js

@@ -841,6 +841,7 @@ const LookingGlass = new Lang.Class({
            this._timeoutId = Mainloop.timeout_add(500, Lang.bind(this, function () {
                 gcIcon.icon_name = 'gnome-fs-trash-full';
                 Mainloop.source_remove(this._timeoutId);
+                return GLib.SOURCE_REMOVE;
            }));
         }));