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/gdm/authPrompt.js

@@ -80,6 +80,7 @@ const AuthPrompt = new Lang.Class({
                                if (event.get_key_symbol() == Clutter.KEY_Escape) {
                                    this.cancel();
                                }
+                               return Clutter.EVENT_PROPAGATE;
                            }));
 
         this._userWell = new St.Bin({ x_fill: true,