New music code revamp in skirmish

[ghost]

The current way that music plays in the background depends on time, so if you were to speed up the game, it would change too quickly. In addition, the sad music plays way too often.

I made some changes to it of my own: https://github.com/FireRainV/Generals-ZH-Mod To make it work better than ever (just look for the MultiplayerScripts.scb.)