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.