Add option to invert Y-axis

[vinnydiehl]

Merged @AdrianFL's changes in preparation for picking #2040 back up. I'd really like to see this QoL feature pushed through.

My plan is to get this usable by correcting the Y-anchoring behavior of rectangles, ellipses, and labels based on the InvertYAxis setting. I've read the other discussions about allowing configurable anchor points for all objects (except I suppose polygons), but IMO that's a separate feature and warrants a separate PR if we ever decided to support it.