Add a clear() method to remove all children

[freakboy3742]

What is the problem or limitation you are having?

At present, you can:

• add a single child to a widget (widget.add(child))
• remove a single child to a widget (widget.remove(child))
• add multiple children to a widget in a single call (widget.add(child1, child2))
• Remove multiple children from a widget in a single call (widget.remove(child1, child2))

However, there is no API to remove all children from a node.

Describe the solution you'd like

widget.clear() should remove all children from a widget. As of Travertino 0.2.0, a clear() API exists; however, invoking it could have unexpected consequences, as it doesn't include the Toga-side cleanups implied by this change.

Describe alternatives you've considered

widget.remove(*widget.children) is a workaround at present, but isn't a great API.

Additional context

No response

[malikoth]

I'd like to tackle this! #pyconsprints2023