Added Edit Models to default scripts

* added editModels.js to defaultScipts.js
* made editModels and editVoxels play nice together for the Delete menu... now they only create a delete menu if one doesn't already exist.
* added features to Menu scripting interface to allow a script to check for existance of menus and menu items.
* fixed a bug in Menu::removeAction() which wasn't cleaning up the action hash.