Add setCentrePosition helper method

I don’t seem to have Actor.setCenterPosition(x, y) so I’ve added in
Position.setCentrePosition(Actor, x, y) instead.

If it turns out I need to update my libgdx we can revert this, however
I couldn’t find it in the javadocs.

Also removed an erroneous import. No idea why these flag up errors for
me, but not for you…