Remove useless IXFObject

Change-Id: I11bdee00bd42ef302dbbfe9f52de5824aec985ee
Remove useless IXFProperty

Change-Id: Id4d5d6aaf537172ad8e1f6c422465c6f84d99dbf
Dead code

Change-Id: Ifdb7c139fa6db7fef2896098f758769cb5284b28
Fix memory leaks, by refcounting XFContent

Change-Id: I8c94c63230eec13bf22043ff07f9f480a0463111