Menubar: move to next item applied focus() to too many items

`_move` was believed to specify just one item,
called `focusableTarget`.  Using the broader
selectors this would return anything matching `a` or
`button`.  This means that the next item _as well
as_ any `button`s or `a`s found thereunder would
_also_ have their focus() event fired.
Menubar: Bug:  Arrow right w/ open submenu error

1.  Load page
1.  Tab to first bar
1.  Right cursor
1.  Down cursor
1.  Right cursor
1.  Error

Test the object that is tested for an `active` for
definition before querying that property.

Create new method for assessing whether or not a
new submenu should be opened. The testing here is
a smell.  This logic should be simplified.
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.
menubar: implement jQuery UI consistency

This code base had been noted for being "unusual"
in comparison with other widgets e.g. `dialog` by
@scottgonzalez et al.  Further drift was
introduced by @sgharms.

This commit reflects the work of bringing
`menubar` closer to `dialog` by means of removing
extremely unusual code invocations and by
implementing feedback from a detailed review by
@jzaefferer.