menubar: Repair TAB order bugs

I believe this fixes tab order.

Regrettably I cannot get the unit test (included
in this commit) which verifies the function of TAB
by use of `simulate` functioning.

The test under investigation is:

"TAB order should be sane mirroring dialog's test "

in `menubar_events.js`.

I'm including it in the PR in hopes that
additional review will present the flaw in my
implementation.  I've tried mirroring the use in
dialog's test suite with no success.

Curiously my use of `simulate` _seems_ right
insofar as my test using it to trigger RIGHT and
LEFT cursor key events _is_ behaving as expected.