Add Lua function: setMusicLoopPoints

[noam-c]

When playing a song, it would be useful to set loop points to make the song loop correctly. In songs with multiple loop sequences, this may mean using a script to change the loop points over time.

This should NOT be used to set the default loop points for a song; that should be encoded elsewhere so that it doesn't need to be repeated every time a script writer wants a song to play and loop correctly.