SVG image plugin does not implement CanRead

According to the comment in the code, it's "hard":

QImageIOPlugin::Capabilities QSvgPlugin::capabilities(QIODevice *device, const QByteArray &format) const
{
    //### canRead disabled for now because it's hard to detect
    //    whether the file is actually svg without parsing it
    //if (device->isReadable() && QSvgIOHandler::canRead(device))

    if (format == "svg")
        return Capabilities(CanRead);
    else
        return 0;
    ...
}

Consequence is that SVGs cannot be read from non-files (eg. QNetworkReply). (QImage has a backdoor QIODevice access to the filename, but fortunately not the QNetworkReply::url().

QDeclarativeImageRequestHandler, and everyone else has to work around this bug. All SVGs start with <?xml ...><comments><svg ...>. This doesn't require "hard" parsing.

The main purpose of CanRead is to detect WHICH handler to use, not to give a totally authoritative "yes this is definitely this format". The PNG loader for example, which claim to be able to read most corrupted PNG files.

Blocks requirement QT-3128.