Change music volumne on Configuration change

[Indy2222]

#521 implements music volume configuration. If the volume changes when the music is already playing, it won't have an effect (until the game restarts). Extend the implementation so that volume change has immediate effect.

This is closely rated to #490.

This can be done by splitting Configuration into multiple independent resources (one resource per configuration section) and using resource_exists_and_changed run condition.