tmm-menubar: can't back out a little

[Dan Jacobson]

M-` (translated from <escape> `) runs the command tmm-menubar

Which is neat, but it doesn't give you a way to back out a little.
E.g., If one types
  <escape> ` o m c
One has no way to back out to the
  <escape> ` o m
level.

It should be associated with the arrow keys perhaps. Like lynx, where
left arrow goes back.

Indeed, left and right arrows currently uselessly just comb over the
prompt in the minibuffer all the way even to column 1.

Left arrow could back out one menu level for each press, right arrow
could offer completions... oh, that is already offered. Hmm.

Up and down arrows, ^P, ^N currently do the correct thing.

Not sure if ^B, ^F should do the same as their arrows.

(Pardon my <escape>, habit from the 1980s.)