decision for whether or not a pane should be drawn out of the layout code and
into the redraw code.
This is needed for the new layout design, getting it in now to make that easier
to work on.
maintain and is only going to get worse as more are used. So instead, add a new
uint64_t member to cmd_entry which is a bitmask of upper and lowercase options
accepted by the command.
This means new single character options can be used without the need to add it
explicitly to the list.
issues - particularly, don't mix with manual pane resizing and be careful when
viewing from multiple clients; generally cycling the layout a few times will
fix most problems. Getting this in for testing while I think about how to deal
with manual mode.
Split window as normal and cycle the layouts with C-b space. Some of the
layouts will work better when swap-pane comes along.
