Add lua support for playing background music

OpenRA.Mods.Common/Scripting/Global/MediaGlobal.cs

@@ -95,6 +95,18 @@ namespace OpenRA.Mods.Common.Scripting
 				playlist.Play(musicInfo);
 		}
 
+		[Desc("Play track defined in music.yaml or map.yaml as background music." +
+			" If music is already playing use Media.StopMusic() to stop it" +
+			" and the background music will start automatically." +
+			" Keep the track empty to disable background music.")]
+		public void SetBackgroundMusic(string track = null)
+		{
+			if (!playlist.IsMusicAvailable)
+				return;
+
+			playlist.SetBackgroundMusic(string.IsNullOrEmpty(track) ? null : world.Map.Rules.Music[track]);
+		}
+
 		[Desc("Stop the current song.")]
 		public void StopMusic()
 		{

OpenRA.Mods.Common/Traits/World/MusicPlaylist.cs

@@ -168,6 +168,14 @@ namespace OpenRA.Mods.Common.Traits
 			Game.Sound.PlayMusicThen(music, onComplete);
 		}
 
+		public void SetBackgroundMusic(MusicInfo music)
+		{
+			currentBackgroundSong = music;
+
+			if (CurrentSongIsBackground)
+				Stop();
+		}
+
 		public MusicInfo GetNextSong()
 		{
 			return GetSong(false);