js: Use EVENT_PROPAGATE/EVENT_STOP constants in event handlers

Just as SOURCE_CONTINUE/SOURCE_REMOVE in source functions, these
constants increase code clarity over plain true/false.

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

js/ui/status/system.js

@@ -81,7 +81,7 @@ const AltSwitcher = new Lang.Class({
                 this._sync();
         }
 
-        return false;
+        return Clutter.EVENT_PROPAGATE;
     },
 });