Easier placement of stages with positioning anchor

[Michael Strauß]

> I also wonder if we want to retrofit that into the centerOnScreen() method (in the future?).
> So that this method will use (set) the underlying Anchor System now instead. So we have only one way to position a Stage.

The new API doesn't allow you to do that quite as easily as
centerOnScreen() does, because you'd have to calculate the center of
the screen first.

I've updated the PR to include Andy's and your suggestions with
additional parameters to control the clamp policy and a "safe zone"
around the screen edges that should be maintained.