treat accessKey as a keyboard event not as a mouse click, fixes #17254
add aria-labelledby to menus spawned by PopupMenu[Bar]Item, fixes #17255
_WidgetsInTemplateMixin: fix conversion error from when deprecated code was removed
various test updates for _WidgetsInTemplateMixin

In particular, use direct test for when startup() gets called, rather than observing
TabContainer behavior.   Refs #9348.

Also don't depend on the click handling being internally implemented as an onClick()
method.
fix Destroyable test to not use dojo.js