Action bars vehicle exit button mover (movable + resizable in unlock mode)

[labrie75]

Makes the vehicle exit button movable and resizable through Unlock Mode.

• New file EllesmereUIVehicleLeave.lua (separate file to stay under the
  200-locals-per-chunk limit); registered in the .toc.
• Drops the forced anchor in FinishSetup so the saved position is used.
• Reuses the original Blizzard art, cropped square with a 1px border.
• Force-shown while Unlock Mode is open so it can be dragged off-vehicle.
• Taint-safe: only methods called on the secure button, combat-guarded.