Bug #5329 : adding a new rules 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)
Empty WYSIWYG documents are considered as non existing on display.

Conflicts:
	lib-core/src/main/java/org/silverpeas/attachment/SimpleDocumentService.java
	lib-core/src/main/java/org/silverpeas/wysiwyg/control/WysiwygController.java
	lib-core/src/test/java/org/silverpeas/wysiwyg/control/WysiwygControllerTest.java
Merge branch 'bug-5329-5.15.x'