Add vehicle "extra" toggles

[gtav-ent]

Use

VEHICLE::SET_VEHICLE_EXTRA
and
VEHICLE::DOES_EXTRA_EXIST

to toggle extras, which apparently include things like lighting rigs. This should be done on the same basis as the skin chooser, i.e. only show options that are appropriate to the target vehicle.

[gtav-ent]

This is added to the vehicle mods menu as toggles. Currently untested.

[gtav-ent]

Working.