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.
