Add focus() method to Selection

[hyuri]

What is the problem or limitation you are having?

The Selection widget currently lacks the focus() method. If the main widget of a page is a Selection widget, it makes it inconvenient that I can't set the focus to it.

Beyond the scope of this report, but this issue seems to also relate to accessibility, as it seems we can't currently navigate/move focus to a Selection using the keyboard.

Describe the solution you'd like

Add focus() method to the Selection widget.

Describe alternatives you've considered

None

Additional context

No response