add getParent() to _WidgetBase

[bill]

Add getParent() to _WidgetBase, but able to find the parent widget regardless of whether or not it extends _Container or sets isContainer.

Since _WidgetBase now supports getChildren(), it makes sense to support getParent() too.

[bill]

In [26531]:

Simplification of startup() code:

1. Remove isContainer: flag and instead make all widgets obey the isContainer:true contract of starting their children.
2. Move _Container.startup() to _WidgetBase.
3. Move _Contained.getParent() to _WidgetBase, alongside getChildren(). But isContainer related code was removed.
4. Removed _Container.destroyDescendants() since it was redundant with _WidgetBase.destroyDescendants()
5. In parser, when deciding which widgets to startup, just check widget._started flag. No need to call getParent(), because if the widget did have a parent, the parent would have called startup() and _started would already be set.

Fixes #13886, #13887 !strict.

[bill]

In [26543]:

remove getParent() calls from dojox/mobile/parser, same as for dojo/parser in [26531], refs #13886, fixes #13892, !strict.