[Amarok] Add enqueue/dequeue shortcuts. BUG: 187542

[Nikolaj Hald Nielsen]

> In general, I agree with you. But there's still some usecase where it's
> useful. If you select multiple tracks and like to enqueue all, where some of
> the tracks are already queued, you need to have two shortcuts.
> But maybe thats a bit over the top.

I think that is a bit of a far fetched use case for having 2 separate shortcuts.

Actually, I just checked, and that behavior is not even well defined
if using the context menu if you have a selection of tracks where some
a queued. The action will always depend on the actual track you are
right clicking on.

- Nikolaj