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
various test updates for _WidgetsInTemplateMixin, _TemplatedMixin

1. Move _WidgetsInTemplateMixin lifecycle test from _TemplatedMixin.html to
_WidgetsInTemplateMixin.html

2. Remove _WidgetsInTemplateMixin tests involving TabContainer behavior since
above test checks startup() call more directly.  Refs #9348.

3. Don't depend on the click handling being internally implemented as an onClick()
method.  Reference click events as "click" rather than "onClick".
Fix regression on IE10 clicking blank screen to close drop down.

An e.preventDefault() call on MSPointerDown prevents the button from getting focus,
so I stopped doing it for that case.  This is a minimal fix.  Other approaches include:

  1. remove the e.preventDefault() call for all cases
  2. make focus manager listen for touch/mouse events in the capture phase rather than the
bubbling phase, to avoid missing the event due to a stopPropagation() call; it used
to do this except for IE6-8, but I normalized the behavior in 9965406.

Fixes #17262.  Refs #13048, #13341.
.gitattributes file to normalize line endings
close dropdown on destroy, fixes #17275
Monitor focus from Menu rather than MenuItem, refs #7111.
This removes the indirect _FocusMixin dependency from MenuItem.