Instead of relying on GScanner and its idea of syntax, code up a parser
that obeys the CSS spec.
This also has the great side effect of reporting correct line numbers
and positions.
Also included is a reorganization of the returned error values. Instead
of error values describing what type of syntax error was returned, the
code just returns SYNTAX_ERROR. Other messages exist for when actual
values don't work or when errors shouldn't be fatal due to backwards
compatibility.
Value parsing only sometimes emitted errors. Sometimes it didn't emit
errors but ignored the value, sometimes it took a default, sometimes it
converted it to something it deemed suitable.
While refactoring, I moved the whole GValue <=> char * conversion
routines to a separate file, to make navigating the core css provider
easier.
