Castbar behaviour in player controlled vehicles

(Title suggested by @haste)

While in a vehicle the player can still cast freely (a good example is opening chests while in a vehicle that allows it).
These commits registers the realUnit for RegisterUnitEvent as well, which lets elements (like the castbar) match the unit arg against self.realUnit.

I proposed this to @haste earlier today, and we decided it's best to put it up as a PR here so you can comment on this change, as we have no real idea if this will break anything :stuck_out_tongue: