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)
Why removing this empty line?
FWIW, it's better to not touch the part you didn't work on (maybe it was added to clearly separate different kind of signals)