UpdateSongPositions

It is useful to have Player SongPositions update off of ScreenGameplay
for use with things like ArrowEffects.

ScreenGameplay, ScreenHowToPlay, and ScreenEdit stop GameSoundManager
from running GameState:UpdateSongPosition and do it themselves, so this
has no impact on the screen classes that cared about, and thus updated,
player SongPositions.
Merge pull request #240 from sigatrev/UpdatePlayerSongPositions

UpdateSongPositions