Summary of the changes/fixes:
1. Trigger checkbox and radio click events identically (cherry-picked from
b442abacbb that was reverted before).
2. Manually trigger a native event before checkbox/radio handlers.
3. Add test coverage for triggering namespaced native-backed events.
4. Propagate extra parameters passed when triggering the click event to
the handlers.
5. Intercept and preserve namespaced native-backed events.
6. Leverage native events for focus and blur.
7. Accept that focusin handlers may fire more than once for now.
Fixes gh-1741
Fixes gh-3423
Fixes gh-3751
Fixes gh-4139
Closes gh-4279
Ref gh-1367
Ref gh-3494
669f720edc