Allow wiki macros to change their macro content during their execution

When being displayed, a wiki macro could be able to change its own macro content in the document in which it is used.

While this feature may look strange at first glance, it can actually be very interesting if we consider the case of documents being edited through WYSIWYG : when starting to edit a document in WYSIWYG, XWiki will render the document in HTML (and thus, will execute the macros of the document). A macro can then wait for its execution in such context, and pre-load its macro content with some content coming from another page of the wiki, or anywhere else. Then, through the wikimacrocontent macro, this pre-loaded content becomes editable in WYSIWYG mode transparently for the user.