Separate the rendering of scales/axes/borders is DataTracks from score rendering

This is so we can overlay tracks, also useful for exome mode

IGV-1979
Re-arrange rendering of border/axis/scales.

Still not perfect in exome mode, gets cut off sometimes. Works for overlay track though.

IGV-1979
Remove redundant isAutoScale method
Have MergedTracks extend DataTrack

This gives us the whole context menu automatically, which we may not want, but we have for now. Implement enough methods that each context menu entry just passes through to the tracks making up a MergedTracks

IGV-1979
Create custom context menu for MergedTracks

Allow user to easily set colors of member tracks individually

IGV-1979
Set alpha=150 for merged tracks.

Have member tracks be removed upon overlay

Add "separate tracks" menu option, which breaks up constituent tracks

IGV-1979