Default player shuffle

This adds some logic to our player selection needed now that we have more than 2 players.

Platform default players can be set, rather than (dvd/pa)player hard-coded.

Also creates a "forbiddenplayer" attribute in playercorefactory.xml, to encourage the creation of rules there rather than hard-coding in various players.

See commit msgs for more specific info.

@davilla @elupus ping.