Added 'editOnce' option to form editors

Also added a stub to extend the onRender function of form views, use
onRenderPlus to do more stuff like adding new buttons to form editors or
on the form itself in module extension.
ToDo::
1. make a tpl for ext.B (module extension type B)
2. consider making the editor options (customized) handler more
generalized (e.g. handle dataSrc option in the same place we handle
editOnce, instead of using the json-def helper converToJSON.js)