This is an initial implementation for `relatedTarget` of `focus` and
`blur` events. As the code doesn't yet follow the specification this is
a bit tricky to associate with specification text, but this should be
correct. A later change will adapt the code to the "focusing steps" part
of the specification.
In addition, a duplicated call to `Element::set_focus_state` is removed.
Testing: This gets one more WPT test passing.
Signed-off-by: Martin Robinson <mrobinson@fastmail.fm>
Co-authored-by: Martin Robinson <mrobinson@fastmail.fm>