add prototypes and use proper type (fn ptr vs void *)

give a name to to the help buffer

pretty print help lines

recompute help only when needed

move help to its own file in order to do so, we also have to export some internal variables from ui (the width and height of the window). Not the best possible outcome, but are used only in recompute_help, and can be dropped later. Also, move wrap_page to wrap.c.