Respect box-sizing when determining the parent component size

[sangaline]

This updates parentSizeObserver() to determine the width and height of the parent component using either box sizing or content sizing depending on the computed box-sizing of the parent. This means that the parent size will include borders and padding when the parent has a box sizing of border-box like is the case by default when the parent is a button.

Closes #86

[kmjennison]

@sangaline This looks great! Thank you for the clear issue and fix.