We don't want to return a GFile because GFile can't handle can't deal
with data: urls.
That makes the code a bit more complicated that doesn't deal with those
URLs, but it makes the other code actually work.
GtkCssImageUrl also now decodes data urls immediately instead of only at
the first load. So don't use data urls if you care about performance.
Emit all errors via the parser, don't try to have a custom error
handling machinery.
The only exception is the initial file load error - we need to do that
one directly, because there is no parser.
This commit is still way too big, but I couldn't make it smaller.
It transitions the old CSS parser to the new parser. CSS parsing is now
tokenized, everything else is probably still buggy.
