The hack in gtk_style_context_get_font() was causing segfaults in
combobox code. This is not acceptable and I'm not awake enough to fix
it, so just reverting until it's fixed sanely is easiest.
This reverts commit cf6bfbdb17.
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.
