Add vehicles back for Wrath

[raethkcj]

I started by reverting 42feaa1644712271eb17ffa4a621e830e021bde5, then added several version checks to try to keep things compatible with TBC/Era.

I'm not sure if adding the Attributes to ShadowedUnitFrames.xml will break in Era or just no-op. If they break, it might need to be split into two xml files loaded by two separate tocs.

I also seem to be missing something with how the toggleForVehicle attribute works; it seems like all of the functions in modules/units.lua are running, and health amount+color are properly swapping. But targeting by clicking and mouseover aren't working, and /dump SUFUnitplayer:GetAttribute("unit") still returns "player" while in a vehicle.

This isn't really a finished PR but hopefully it's a good starting point, as having vehicles working properly is pretty essential in Wrath.

[Brixus3]

Please merge. SUF is unusable for wrath in its current state.

[raethkcj]

With https://github.com/Stanzilla/WoWUIBugs/issues/274 fixed, these changes are fully working on the 3.4.1 PTR and are compatible with Era @Nevcairiel.