No music in observermode

[elexis]

Since the music tracks are categorized by civ, observers do not experience any audio track currently.

One solution would be to add a set of tracks to observers (that fit to all civs).

Another idea might be to play the track of the most recently selected civ and change the trac when picking another. To avoid restarting the track all the time when switching the perspective, all tracks of all players could play continuously, but ony one of them having the actual volume. Thus the tracks could also be faded.