[w3c/uievents] Need algorithm for how focus events are fired (#195)

We need a algorithmic description of how focus events are fired.

`blur`, `focus`, `focusin`, `focusout`

It should also cover: `DOMFocusIn` and `DOMFocusOut`

Starting point:

```
<User shifts focus>
1 focusin Sent before first target element receives focus
2 focus Sent after first target element receives focus
3 DOMFocusIn (If supported)
<User shifts focus>
4 focusout Sent before first target element loses focus
5 focusin Sent before second target element receives focus
6 blur Sent after first target element loses focus
7 DOMFocusOut (If supported)
8 focus Sent after second target element receives focus
9 DOMFocusIn (If supported)
```

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