Undeprecate (precate?) the !LayoutContainer code, and make !BorderContainer extend !LayoutContainer.  Fixes #16770 !strict.   Will add reference doc too.
Handle "leading" and "trailing" regions in !LayoutContainer rather than !BorderContainer.   Also handle setting of classes on children widgets there.  Refs #16770 !strict.
avoid spurious error about region being undefined, refs #16770
Fix data-dojo-attach-event on widgets (as opposed to DOMNodes), fixes #16798,  refs #16585 !strict.
Move _WidgetsInTemplateMixin specific code from _AttachMixin to _WidgetsInTemplateMixin.  Refs #16576 tangentially, !strict.
Merge remote-tracking branch 'upstream/master' into custom2