[libtasque] Improve BackendTaskCollection
[libtasque] Improve BackendTaskCollection

* initially register categories' changed events
* on dispose: unregister from source collectino changed event
* some formatting
* AddColllection and RemoveCollection dropped. No need for them as
source collection is maintained by another object
* On source collection changed: register/unregister changed events of
added/removed category
mono-soc-2012/Tasque,mono-soc-2012/Tasque,mono-soc-2012/Tasque