DOM like API

[patrickbkr]

I'd like to have a way to change the widget layout dynamically.

My use-case would be to implement a tab-page like behavior in the debugger I'm working on. There are more details of how it should look in #11. "Tabs" I'd like to show / hide are:

• Default view
• Settings
• Help
• Thread selection (Keeps the breadcrumbs at the top)
• Frame selection (Keeps the breadcrumbs at the top)

There are surely other ways to do this (e.g. always adding all widgets to the surrounding container and hiding some of them), but an API to manipulate the widget tree feels like a good low level mechanism to have in many respects.