Overlays should support vertical and horizontal alignment

[ecton]

The Menu widget currently works ok, but when a menu opens at the edge of the screen, the OverlayLayer shifts the widget inward, covering the parent menu. When using relative directional placement, we do intersection detection, preventing this from happening when possible.

When using the at positioning, we skip doing any intersection detection because we don't have any fallback strategies. Ideally, we could support a "align this to the right or left, but use this y coordinate" type positioning. When thinking of this and how toasts might be positioned, this seems to make a lot of sense.

I think the current approach to Direction + Relative or Absolute isn't quite the correct combination, and there might be a better way to express these different positioning strategies.