fixing bugs #2690 and #5617 :
- preventing exception when trying to remove a node which not exist in JCR (SimpleDocumentInstanciator.java)
- adding new method to remove main app's index directory
- adding and using new method to delete all files on server when an app is definitively removed (Admin.java)
- thumbnail was not removed from file server (bad subdirectory name)
- removing some useless methods in FileRepositoryManager.java
fixing bug #5721 : adding scrollbars to window
fixing bug #5827
Fixed bug #5845 (Aurore's work)

Conflicts:
	war-core/src/main/webapp/util/styleSheets/fieldset.css
Fixing a XML/HTML content rendering of the WYSIWYG editor (Aurore's work)

Conflicts:
	war-core/src/main/webapp/wysiwyg/jsp/htmlEditor.jsp
Merge branch 'master' of github.com:Silverpeas/Silverpeas-Core into feature-5519