Use server-time times, if possible, when producing logfile timestamps

This adds a "dateTime" argument to ChatWindow::logText, and also puts this information in IRCView::textToLog. It splits off IRCView::timeStamp's server-time extraction into its own function, IRCView::messageDateTime, and uses this in the various functions that emit textToLog.

I've tried to do this properly, and I think this is a useful change (it is for me), so please tell me if I've done something wrong and I'll try to fix it. Thanks.