Add shortcut that jumps to the markerline

By default (and subject to change), Ctrl+K now jumps to the markerline
(which is either set manually by Ctrl+R, or by default automatically when
switching channels).

If necessary, additional backlog will be fetched first. Note that this might require
multiple presses of Ctrl+K if you tinkered with message filters after setting the markerline,
and also for markerlines that have been set using older clients.

The markerline will now never be set on a day change message.