Use layout to resolve width and other CSS properties

[Jym77]

Several style properties compute their percentage values depending on the size of the containing box. So far we couldn't do anything about it and simply forwarded the percentages as computed values.

With the layout system coming up, we will be able to actually fetch the size of the box and resolve these percentages, thus resulting in more accurate computed values that can be used, e.g. in R83 to compare elements' sizes.

This will likely require some overhaul of the style system in order to forward the element's box size all the way to Style#computed.