Added support to cross-fade some battle music when encountering enemies.

As client game map info parameters were loaded after the game mode was activated
due my recent change about loading the level only once the server tells which to load,
I had to fix the game mode to activate only after the client game map level loading.

This fixed the calm music not played anymore when starting the game.

I also made the server tell the specific client when attacked or attacking because
the attack and damage taking logic was only done on server-side.

Fixes #23