Si(pheight) and si(pwidth)

Hey Frank,

I was hoping to get a function to easily use the parent containers width and height in a formula for things inside that group. Example:

$Si(pwidth)/2$ could make the width of a shape half of the width of its parent group.

To afd another use case:

Outer Group

>Outer Border shape



Outer border shape would need to grow to match outer group as it accomodates Hstack.

Either that or groups should have a border tab added to properties. All for that.