Improve compatibility with the subsonic api

This seems to be how its supposed to be implemented

set - should only change current playlist and shouldnt modify index

This lets dsub work properly when it comes to modifying the current
playlist as it does a combination of "set" and "skip"

Itll only use set if its adding tracks to the playlist and includes
the whole playlist, and itll use both set and skip to set the current
playing and skip back to the first track

server/jukebox/jukebox.go

@@ -96,20 +96,8 @@ func (j *Jukebox) doUpdate(u update) {
 	j.Lock()
 	switch u.action {
 	case set:
-		if j.playing {
-			speaker.Clear()
-		}
-		speaker.Clear()
-		if len(u.tracks) == 0 {
-			j.playlist = []*db.Track{}
-			j.Unlock()
-			return
-		}
 		j.playlist = u.tracks
-		j.index = 0
-		j.playing = true
 		j.Unlock()
-		j.speaker <- updateSpeaker{j.index}
 	case clear:
 		speaker.Clear()
 		j.playing = false
@@ -118,12 +106,23 @@ func (j *Jukebox) doUpdate(u update) {
 	case skip:
 		speaker.Clear()
 		j.index = u.index
+		j.playing = true
 		j.Unlock()
 		j.speaker <- updateSpeaker{j.index}
 	case add:
+		if len(j.playlist) == 0 {
+			j.playlist = u.tracks
+			j.playing = true
+			j.index = 0
+			j.Unlock()
+			j.speaker <- updateSpeaker{0}
+			return
+		}
 		j.playlist = append(j.playlist, u.tracks...)
+		j.Unlock()
 	case remove:
-		if u.index < 0 || u.index > len(j.playlist) {
+		if u.index < 0 || u.index >= len(j.playlist) {
+			j.Unlock()
 			return
 		}
 		j.playlist = append(j.playlist[:u.index], j.playlist[u.index+1:]...)
@@ -144,10 +143,15 @@ func (j *Jukebox) doUpdate(u update) {
 }
 
 func (j *Jukebox) doUpdateSpeaker(su updateSpeaker) error {
-	if su.index > len(j.playlist)-1 {
+	if su.index >= len(j.playlist) {
+		j.Lock()
 		j.playing = false
+		j.Unlock()
 		return nil
 	}
+	j.Lock()
+	j.index = su.index
+	j.Unlock()
 	f, err := os.Open(path.Join(
 		j.musicPath,
 		j.playlist[su.index].RelPath(),