macosx: Display A to B loop marks in playback progress slider

This provides some sort of feedback ot the user when they start setting A->B marks, as previously we wouldn't display anything at all anywhere.

Also includes some cleaning up (and renaming) of the VLCSlider (now VLCPlaybackProgressSlider) class

Signed-off-by: Claudio Cambra developer@claudiocambra.com