Factory refurbished PlayMusicOnMapLoad

To fix https://github.com/OpenRA/OpenRA/issues/6071 and bring it in line with the legacy https://github.com/OpenRA/OpenRA/blob/bleed/mods/common/lua/media.lua#L11 which this aims to supersede.
I think the name `Loop` is misleading, especially now that you're introducing a `Repeat` attribute as well. How about reversing its meaning and calling it `StopAfterSong` oder `PlayOnce` or something?