Bugfix amarok crash

[Ralf Engels]

...
>     * This patch is going to make "remove tracks" agonizingly slow in
>       large playlists. O(N*M).
> 

I haven't thought about this but O(N*M) only holds if you clear the
whole playlist.
For deleting a single file with O(N) that should be fine.

> Also: 'Playlist::Model::clearCommand()' fails to do the necessary 
> 'unsubscribeFrom()' calls.
> 

I haven't noticed that.


Still not Amarok crashes with the patch where I had around three per day
without it.

Ralf