move the "ready to start" sound to the "turn bell" channel

This allows us to play all the sounds correctly in mp without
needing more than two UI channels.

The second UI channel was created in this commit:
e39797b6ad615d4f619751a6ec6510ca716f5811

Together these fix a problem reported here:
http://forums.wesnoth.org/viewtopic.php?f=4&t=41059#p576534
update changelog