Editors: Make sure folded blocks remain consistent

When folding indents change and a block becomes no longer
folded we need to update the user data. This patch tries
to handle general inconsistencies that might arise in such
situations. Notice however that there are stil other problems
to be addressed (including issues in Qt).

Task-number: QTCREATORBUG-5771
Change-Id: I38b869832159598d46cde00058308c218ca31f1a
Reviewed-on: http://codereview.qt.nokia.com/2908


Reviewed-by: Robert Löhning <robert.loehning@nokia.com>
Reviewed-by: Matthias Ettrich