Fix #13353; #13428; #14359: Better treatment of native-backed events (focus/blur/checkbox-click)

It's big, but much better than https://github.com/gibson042/jquery/commit/80789b1ced9ffd45b497e971978d81f847600bec... and this brings the count of fixed bugs up to 3, further reducing the added size per fix! :wink:

At any rate, I also found some possible optimizations in event.js that I'll be exploring once this PR closes either way.

   raw     gz Compared to 1.x-master @ fd2964237f07d8b6985df2145a4990f96eddba87
```
 +2073   +586 dist/jquery.js
  +484   +103 dist/jquery.min.js
```