Implement object/page deletion.

Currently we don't support object deletion, just cretion and modification.
Deletion is only supported at wootable content level by the WootEngine component.
Also, the ContentProvider component does not detect/notify of delete operations on the XWiki side.

To implement object deletion, we would first need the ContentProvider to notify XWoot of deletions as well, by returning XWoot objects with a metadata flag, marking that an object was deleted.

When receiving such an XWootObject, the Thomas-rule engine should mark it as deleted by setting the tombstone to true and then let the WootEngine mark all visible WootRows as deleted as well, for all the wootable fields of that object that it manages.

A patch will be generated with the above modifications (if the XWootObject was received from a synchroization with the XWiki) and propagated to the other peers.

If a patch is received for a deleted object, the object must be un-deleted. Thus, the TRE engine must set the tombstone to false for that object and, together with the WootEngine, must apply the operations described by that patch.