SoundEmitter: Optimize source allocation strategy

Before, we would blindly rotate through the sources (like a
revolver through its chambers), which worked great if one
assumed all sounds to be relatively short and therefore oldest
use == most likely to be free, but breaks if there is one long
sound playing, which would be stopped and overtaken if we rotated
back to it even though there might be other free sources available.

Instead, keep an ascending priority list of sources with last
used == highest priorty that is iterated through for the first
free one, and only if none is found overtake the one with lowest
priority. This also ensures we're always able to play 'SE_SOURCES'
sounds at once independently of their length.

Fixes #37.