Add option for chat monitor to use backlog

Will show valid messages from the backlog in the chat monitor if the "Show messages from backlog" option is set in the Chat Monitor configuration page.

The second commit adds an option to include read messages in the chat monitor when it uses the backlog.

As per the comments on https://github.com/quassel/quassel/pull/44 I've applied the patch linked from http://bugs.quassel-irc.org/issues/734 directly, then fixed the merge conflicts caused due to style changes.