Nepomuk tag provider implementation:
- tested manually and basically works
- as Nepomuk lacks support for resource monitoring the provider can't detect when a tag has been added/modified/deleted. It can only detect changes made by the application itself.
- it allocates a new Nepomuk resource for a feed/item when a Nepomuk tag gets attached to this feed/item
- it breaks FeedListModel because the signals from Akonadi and the Nepomuk tag provider (when deleting a tag) arrive unordered (the fix is coming)

Not recommended until Nepomuk gets support for resource monitoring.
Bugs and spaghetti code included.