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.
Instead of an error vfunc, have the tokenizer vfunc take a GError
argument. Note that even when an error is returned, there is still a
token to be read.
This library is meant to be the new CSS library that gets used from GDK,
GSK and GTK for string printing and parsing.
As a first step, move GtkCssProviderError into it.
While doing so, split it into GtkCssParserError (for critical problems)
and GtkCssParserWarning (for non-critical problems).
