GError-specific logic could be easier

Firstly, if I've missed some more convenient API here, I apologise.

I've just spent the last quite-a-while trying to figure out how to handle matching GErrors within a specific domain. While the GError objects themselves helpfully expose .domain, finding a constant that maps to that turned out to be challenging.

What I ended up with (after reading a bit of the pygobject source) was basically:

    try:
        # do my dodgy unicode-decoding thing
    except GLib.Error as err:
        if err.matches(GLib.convert_error_quark(), GLib.ConvertError.ILLEGAL_SEQUENCE):
            # handle stuff

The main reason that this process ended up being so confusing was that I couldn't find anywhere in Python land where I could reference the string constant in the err.domain attribute.

IMO, a significant improvement would be to expose the quark and/or domain on the individual GLib.Error subclasses. Having said that, at that point there's no reason that matches couldn't just take the enum and do the quark check from that.