Improve internal box model terminology

[alexreardon]

Currently we use the following language to describe different areas associated with a dimension:

• withoutMargin: straight from getBoundingClientRect
• withMargin: we use window.getComputedStyles(el) to pull the margin from the element it and add it to withoutMargin
• withoutPadding: we use window.getComputedStyles(el) to obtain the elements padding and then we shrink the withoutMargin dimension by this amount.

These terms have grown organically as needs arose. They also directly map to the box model. To reduce friction we should rename our properties to match their box model equivalents

• withMargin -> marginBox
• withoutMargin -> paddingBox
• withoutPadding -> contentBox

For now we are getting away with ignoring borders 😊 but we could look at adding a borderBox as well. Although, we could do this as a seperate task