FR: Increase flexibility of Music.tbl

The current music.tbl only supports contextually switching between one type of track (Battle) when the player joins combat and one (Normal) for everything else. It would be very helpful in adapting the FotG music to FSO if we could expand the conditions that triggered track changes in order to more smoothly follow the flow of a mission. Just having an "Alert" state for when enemies are on radar but not engaged would make a major difference in selling the illusion of a deliberately-cued musical experience. Other divisions like different sets of battle tracks for different scales of engagement could also be very useful. Multiple slots for each type of track (in the same way that Battle is split in three) could also be quite helpful!

@wookieejedi looked into implementing this type of functionality via scripting but determined that getting a smoothly functioning system comparable to music.tbl in that way was probably not gonna happen.