Turns out we still need more in the horizontal due to cutout margins at
the bottom of the slot. This isn't needed in the vertical. Separating
these lets us give more horizontal gap again without sacrificing tight
vertical tolerancees.
Shrink the button size so it fits inside the outer divet with a margin
of PartMargin, rather than giving both the same radius.
Cut off the button sphere at a more accurate position in case the tab
thickness is smaller than half the button radius.
The panel gap didn't need to be as large around the outer edge, so call
that PanelEdgeGap and default it to PartMargin*2. Leave the thickness
margin at CutoutMargin*2 + PartMargin*2 and rename it to PanelThickGap.
