Music track should not restart when switching the menu

[elexis]

The currently played track is restarted when entering the multiplayer lobby, or when returning from any page to the main menu (whenever global.music.setState is called in the JS files).

The number of resets should be minimized - it is basically only needed when first starting the game and when leaving the summary screen after ending a game (the summary screen should still play the win / defeat music of the players civ).