emacs: jump: make multilevel keys do multilevel jump

emacs: jump: make multilevel keys do multilevel jump

notmuch jump allows the user to specify a key sequence rather than
just a single key for its bindings. However, it doesn't show what has
already been typed so it can be difficult to see what has
happened. This makes each key press appear, and the jump menu reduce
to the possible follow up keys.

We also bind backspace (emacs symbol DEL) to go back up a level in the
subjumpmaps, and to exit from the top level.