simplify startup() code

[bill]

Simplify 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. 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.

Also, remove _Container.destroyDescendants() since it's redundant with _WidgetBase.destroyDescendants().

[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 [26740]:

Fix problems where ContentPane was not calling startup() on non-widget children. Fixes #13971, refs #13887 !strict.

[bill]

In [26762]:

At the time that submitButton.startup() is called, form.isValid() isn't ready to give the right answer. Switch form.isValid() call to run after form.startup() completes. Refs #13971, #13887.