Improve scaffolding to bind Helper class to execute common code (instead of limited renderEditor.template)

The current Scaffolding generator binds the renderEditor.template when generating views.
This template is called from the create and edit gsp's.

This functionality is limited to share common code for editors and it's not possible to have other common code in the generation process.

In the XTemplates plugin I had solved this limitiion by binding a Helper.groovy class instead of the renderEditor.template in both the generateView and generateController process.
This makes it's easy to have common code to be called when generating views and controller.

I've attached a small patch for this.
New file: Helper.groovy
Changed files: create.gsp and edit.gsp
Obsolete file: renderEditor.template

I've tested it with both static and dynamic scaffolding in dev and war mode