Return true if the element can be focused.
An element can be focused if it fulfills all the following conditions:
- It's focusable. See Focusable.focusable.
- It's enabled. See VisualElement.enabledInHierarchy.
- It's visible. See VisualElement.visible.
- Its display style is not DisplayStyle.None.
- If it has a parent that delegates focus (see Focusable.delegatesFocus),
then that parent needs to also fulfill these conditions.
Additional resources: Focusable.focusable, VisualElement.enabledInHierarchy, VisualElement.visible, IStyle.display, Focusable.delegatesFocus, Focusable.Focus