Richtext Xinha package revamp:
since this editor now seems to enjoy somewhat better support, I have "borrowed" much of the logic available in richtext-ckeditor4 so that users can keep their editor version up to date.
Notable changes:
- we do not ship the actual Xinha editor ourselves, it will by default come from the cdn (on Amazon servers) or can be downloaded locally as we do for CKEditor4
- plugins that have been developed specifically for OpenACS (which seem to keep working) are now maintained in a separate folder tree and configured in the editor specifically
- The configuration for which plugins should be available is now maintained at the package level
