Add "reverse gear" option for ground units in mission editor

[Mad_Shell]

Currently it is impossible to make a ground unit use reverse gear. It's especially limiting when trying to make realistic tank behaviour, since they use it a lot for masking/unmasking

[Mars Exulte]

I think the issue is trying to emulate complex intelligent behavior through hamfisted scripting and micromanaging. Much more practical to ask for AI in general, which ground units almost entirely lack.