See inline code comments taken from
http://dev.w3.org/csswg/css3-cascade/#cascade
This now respects the special values "inherit" and "initial" properly.
Note that those values cannot be parsed yet. This will be added in a
future commit.
We now use the GtkStleProviderPrivate interface, which hopefully is
faster and more conformant to CSS. Long term, it definitely should be
both.
I would have liked to split this up into multiple commits, but couldn't
find a way.