Use the headerbar for controls

[fossfreedom]

Well it seems upstream are having difficulties with accepting the headerbar approach for controls.

We can implement that approach fairly easily.

So it might be nice to make this an option.

It is basically the current bottom bar but put into the headerbar. Leave out the source toolbar movement stuff.
The slider should be available via a slimline toggle button next to the coverart at a guess

So could just steal the key bits of the headerbar code