Better support for number of groups > terminal height

When using mcjoin today with a large number of groups, where num > terminal height, the output is not pretty.

Ideally, only the first (height - num) groups should be shown and you could use PgDn/PgUp to scroll through them.  The PgDn/PgUp binding is reserved for the log buffer atm., but that could be handled by allowing the use of Tab to move the view between the two panes.