Qt: Gray out "New multiplayer window" after four windows

[tony971]

It's already disabled. There's just no visual representation of it being disabled.

[waddlesplash]

Can I take a stab at this?

[endrift]

Yeah, sure! Two things to take note of:

1. There's a MAX_GBAS macro floating around, I think it's in sio.h
2. The wireless adapter supports more than 4 systems, so this might need to be reworked later.