Implement MafiaTest developer menu

[zpl-zak]

A simple imgui menu containing all sorts of functions to make development and playtesting easier.

Features included:

• mission selector
• object spawner
• player cheats (god mode, infinite ammo, speedy gonzales, ...)
• particle spawner
• car spawner
• car cheats (repair car, refuel, set damage level)
• police cheats (set wanted level, ...)
• AI cheats (spawn bodyguard, spawn enemy, gang wars, ...)