Focused nodes and focus owners

[Michael Strauß]

Scene graph nodes have three focus-related properties:
1. focused
2. focus-within
3. focus-visible

What might be a bit surprising is that multiple nodes can be focused
at the same time, but only one of those focused nodes can be the
scene's focus owner. The `Scene.focusOwner` property indicates which
of the focused nodes is the current focus owner.

Even more surprisingly, a node can be _unfocused_  and be the focus
owner at the same time (this can happen when the window is
deactivated).

>From the perspective of `Node`, there's currently no API to easily
determine whether it is the focus owner, and there's no CSS
pseudo-class that matches a node that is the focus owner.

We could add two more properties to the `Node` class to support this use case:
4. focus-owner
5. focus-owner-within

Note that `focus-visible` only matches nodes that are also focus
owners, so there's no need to have a `focus-owner-visible` property.

The two additional properties would suffice to match any possible
focus state, but it might also be not useful enough to warrant the
additional API surface. What do you think?