Add RelativeAlignX and RelativeAlignY

[nreagan]

RIght now, HorizontalAlignment and VerticalAlignment have conflated use cases. Looking at just horizontal,

1. If HorizontalAlign is set, and X is set, the element will align itself to put it's local body aligned with the X
2. If HorizontalAlign is set, and X is not set, the element will align itself with the parent's X dimensions

This is conflating 2 different concepts of alignment, one local and one relative. Want to separate the two.