Add support for a single "marked pane". There is one marked pane in the

Add support for a single "marked pane". There is one marked pane in the
server at a time; it may be toggled or cleared with select-pane -m and
-M (the border is highlighted). A new target '~' or '{marked}' specifies
the marked pane to commands and it is the default target for the
swap-pane and join-pane -s flag (this makes them much simpler to use -
mark the source pane and then change to the target pane to run swapp or
joinp).