Fixed bug #3556715 (http://bit.ly/O3QN0u) where returning to a modified or deleted snippet or category in history results in GPF.
Fixed bug #3557107 (http://bit.ly/PfiYxm) where new snippets and categories were not being included in history.
Implemented by adding database change listener and acting on notified changes - modification and deletions cause history to be cleared and additions cause a suitable view to be added to history.
