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.
I made some mistakes in the selector parsing code. It's now able to
parse selectors composed of multiple complex selectors, instead of just
one complex selector.
This currently returns a JS::Array of elements matching a selector.
The more correct behavior would be to return a static NodeList, but as
we don't have NodeLists right now, that'll be a task for the future.
