Add Q_DECLARE_METATYPE(IrcWhoReplyMessage*) for QML compat
Kill IrcWhoReplyMessage::userName/userHost/nickName

Use IrcMessage::nick/ident/host instead
Rename IrcWhoReplyMessage::channel to "mask"

It's not necessary a channel, it could be any sort of mask with optional
wildcards. Thus, use "mask" that matches IrcCommand::createWho(mask).
Fix IrcBufferModelPrivate::messageFilter()