In-Game HUD: Weapon Switch

# Weapon Switch HUD

We need to implement an in-game weapon switching HUD!

The style is undetermined, however a weapon wheel would be nice for quick access, especially on controllers. For backwards compat with HL2, we can implement an alternative hl2-style mode as well, although that's low priority in this instance.

Implementation of this HUD element relies on #42 being finished.