This moves Servo closer to the focus parts of the HTML specification.
The behavior should be the same as before, but now the code in `script`
matches the structure of the specification.
The main goal is to set us up for:
- Firing focus events in the right order on nested documents
- A proper implementation of the unfocusing steps.
Testing: This should not change behavior so is covered by existing
tests.
Signed-off-by: Martin Robinson <mrobinson@fastmail.fm>
Co-authored-by: Martin Robinson <mrobinson@fastmail.fm>