Enable codemirror diff view for web pages
ERP5's Code mirror supports viewing history of editions using codemirror's merge mode ( http://codemirror.net/demo/merge.html ).
This was enabled for portal_components, but not for web pages.
This moves the methods used by the components in a mixin class and make TextDocument class use this mixin.
This way, we can benefit from diff view on web pages as well:
@romain @vincentB @katomaso unless this interfere with your work of making codemirror works in renderjs ui, I think we can enable this. It does not include any change to codemirror code, just enable everywhere what was available only on components.
@klaus I did not check, but we probably want this enabled for PyData Script as well.