Display track color in icon in menu

IGV-1979
When tracks are merged/separated, put the result in the same position as the first track

When overlaying an overlay track (sup dawg), separate the member tracks so we don't have nested MergedTracks

Note to self: Use more consistent naming if possible

IGV-1979
Add ability to save/load MergedTracks to session files.

Very hacky right now, since MergedTracks are the only kind which can have tracks as children and those tracks need to be associated with resources before being put in the MergedTracks instance

IGV-1979
Add some comments about possible future refactoring
IGV-1979
Merge remote-tracking branch: upstream/master