'panels' library almost completely rewritten. The new version is

_much_ better at figuring out which parts of the screen need to be
updated,  aided in part by a new PDC_touched_range() function to
determine which parts of a window have changed.  The panels are
stored in a circularly-linked list,  of which the stdscr pseudo
panel is always a member,  so edge cases are mostly eliminated.
Panel overlap is computed in a less confusing manner.