Sound: Context music with seamless transitions

[DirectuX]

• Beneficing from the new ecs pattern, we could have events like "entering this gameZone" that triggers the music transition. A short delay can be implemented to counter an untimely change of the background music at zone frontier.
• Same pattern can be used for other events like "a level has been unlocked" "a new skill has been acquired".
• Events like "more than n foes in the reach of the Pilgrim" "no more foes around" could add more immersion.

[petrSchreiber]

Superb, moving to accepted.