fix: Node::is_focusable always returns true if the node is focused

[mwcampbell]

I think it's reasonable to only add a given action to a node if that action does something meaningful in the node's current state. And we may want to recommend that. But things like the UIA IsKeyboardFocusable property and the AT-SPI focusable state should be set even when the node is already focused, so the node might not have the Focus action in that state.

One subtlety: I added a helper, is_focused_in_tree, to check whether the node is focused within the tree, regardless of whether the host (e.g. window) is focused. If the node is focused in the tree, then the node might not have the Focus action, regardless of whether the host is focused.