fade: Switch to the fade-in mood *before* sleeping.

fade: Switch to the fade-in mood *before* sleeping.

This way the fade-in mood is active when para_fade is interrupted
during the sleep.

Currently sweet_dreams() calls change_afs_mode_and_play(),
which switches to a different mood and changes the mood directly
thereafter. For the new sematics we need to separate the two
actions though. Hence change_afs_mode_and_play() is renamed to
change_afs_mode(), and it now performs the mood switching part
only. Its two callers are modified to call client_cmd("play")
themselves.

This allows to call change_afs_mode() before we go to sleep and
client_cmd("play") on wake up.