Bug #5329 : adding a new rule around WYSIWYG attachment management.

When updating a WYSIWYG attachment for a language with an empty content, the content for language is not saved but deleted. After that, if no content for any language exists for the attachment, then it is deleted from the JCR.
(adding some unit tests around the WYSIWYG controller)
(adding a List Object for SimpleDocument to centralize common behaviors)

It could be useful to integrate in a first time https://github.com/Silverpeas/Silverpeas-Core/pull/506 PR before this one, because of all fixes it brings around the versioning management, especially around multi-lingual content.

There will be a manual merge operation into the master version of Silverpeas-Core around the WysiwygController class. Indeed, the class has been deeply modified during the developments made for the new application: suggestion box. Thank you so to not hesitate to get back to me in case of difficulties.