Add clear queue button

[Oowoosh0]

Adds a button to remove all elements from the queue.
One of the features requested in #779

[Oowoosh0]

Just noticed that it would make sense to hide the button if the queue is already empty. I'm doing to add that.

[danirabbit]

hide the button if the queue is already empty

@Oowoosh0 Instead of this, you should disable the action and it'll automatically make the button insensitive and ignore any future keyboard shortcuts :)

[Oowoosh0]

the action is now disabled when no items are in the queue.
The disabled buttons background is automatically slightly darker and has an inset box-shadow. Even though the style class is flat. I don't know if that is intended by the elementary stylesheet or if that is a bug. I think the box-shadow looks a bit odd in dark-mode.

[danirabbit]

Looks good!

I see what you mean with the styling. That's definitely a stylesheet bug and not anything for you to fix here