This patch introduces support for more than just "absolute px" units in
our Length class. It now also supports "em" and "rem", which are units
relative to the font-size of the current layout node and the <html>
element's layout node respectively.
Many properties can now have percentage values that get resolved in
layout. The reference value (what is this a percentage *of*?) differs
per property, so I've added a helper where you provide a reference
value as an added parameter to the existing length_or_fallback().
This momentarily handles the CSS property "position: absolute;" in
combination with the properties "top" and "left", so that elements can
be placed anywhere on the page independently from their parents.
Statically positioned elements ignore absolute positioned elements when
calculating their position as they don't take up space.
