The comments were copied from the macOS SDK header file and explain
what the functions do, which does not require such a long comment and
is detailed in Apples documentation of NSSplitView too.

Remove unused VLCProgressView

Remove unused MPSlider class

This avoids bogus return-type warnings.

This was semi-broken and is no longer necessary.

When nothing is playing, the knob and progress should not be shown,
this adds an option for this to the VLCSlider

(easier to test separately)

This reverts commit dcd3a45e.