big commit, with lots of changes/features. here is a summary:

* created new items/ directory for ContextView items and related classes
* added Observer pattern for use with ContextView items. items now connect as observers to the ContextView.
it is important that items do *not* directly connect to EngineController because ContextView must be run before any items.
* added Context Item Manager, available from the Tools menu. it is now possible to enable/disable context items (changes come into effect on next track
change). it will soon be possible to control the "order" of items on the CV in the manager.
* items are no longer singletons ( except LyricsItem, needed for dbus handler )
* CloudBox is now a member of LastFmItem (more logical place for it).
* i think thats it.