Addon improvements

As discussed on the forums, this does three things:

1. Reduce to a single notification during repo updates if multiple add-ons need updating.
2. Don't categorise add-ons under the Disabled Add-ons and Available Updates items in the add-on manager.
3. Adds an Update all item to Available Updates.

@elupus to review the busy dialog change.  I suspect this can also be used (perhaps with modification) in CUPNPPlayer, and possibly also (again with modification) in the directory fetcher.  With a general function we may also need to consider how to handle non-app-thread updates (atm all of them are on app-thread I think).