Fix for queue ordering issue in the playlist view when using c-d to dequeue a track (issue #4015)

Fix for the queue order problem referred in issue #4015.
When dequeuing a track the executing function wouldn't update the remaining tracks in queue. Now it updates the queuing position of the remaining tracks instantly.
To do that I created a signal that allows to announce the change in the queue (QueueChange).

In the playlist view created a slot that updates the whole view (as qt optimizes it) as discussed in [1], which may be used by other procedures or events if necessary.

That's it. Waiting for feedback.
Thanks!

[1] - [Forum link](https://groups.google.com/forum/?hl=en#!topic/clementine-player/XMLstJKP0rk)
OK :) I wasn't sure if it was something like this or because you simply missed that connecting to update() directly should be possible.
I'd rather get rid of this intermediate UpdateView method. "UpdateView" sounds like it is updating the view, and calling PlalistView::update() does this, so it sounds unnecessary to have an intermediate method to make the code clearer.
About abstracting, I'm not sure it is useful here, because I'm not sure we would need to have our own update method for this kind of things oneday.

So if you don't mind changing this, I would be glad to merge your patch :)