If during a focus handler another focus event is triggered:

```js
elem1.on( "focus", function() {
	elem2.trigger( "focus" );
} );
```

due to their synchronous nature everywhere outside of IE the hack added in
gh-4279 to leverage native events causes the native `.focus()` method to be
called last for the initial element, making it steal the focus back. Since
the native method is already being called in `leverageNative`, we can skip that
final call.

This aligns with changes to the `_default` method for the `click` event that
were added when `leverageNative` was introduced there.

A side effect of this change is that now `focusin` will only propagate to the
document for the last focused element. This is a change in behavior but it also
aligns us better with how this works with native methods.

Fixes gh-4382
Closes gh-4813
Ref gh-4279

(cherry picked from commit dbcffb3)

The `_default` function in the special event settings for focus/blur has
always returned `true` since jquerygh-4813 as the event was already being fired
from `leverageNative`. However, that only works if there's an active handler
on that element; this made a quick consecutive call:

```js
elem.on( "focus", function() {} ).off( "focus" );
```

make subsequent `.trigger( "focus" )` calls to not do any triggering.

The solution, already used in a similar `_default` method for the `click` event,
is to check for the `dataPriv` entry on the element for the focus event
(similarly for blur).

Fixes jquerygh-4867

The `_default` function in the special event settings for focus/blur has
always returned `true` since gh-4813 as the event was already being fired
from `leverageNative`. However, that only works if there's an active handler
on that element; this made a quick consecutive call:

```js
elem.on( "focus", function() {} ).off( "focus" );
```

make subsequent `.trigger( "focus" )` calls to not do any triggering.

The solution, already used in a similar `_default` method for the `click` event,
is to check for the `dataPriv` entry on the element for the focus event
(similarly for blur).

Fixes gh-4867
Closes gh-4885

The `_default` function in the special event settings for focus/blur has
always returned `true` since jquerygh-4813 as the event was already being fired
from `leverageNative`. However, that only works if there's an active handler
on that element; this made a quick consecutive call:

```js
elem.on( "focus", function() {} ).off( "focus" );
```

make subsequent `.trigger( "focus" )` calls to not do any triggering.

The solution, already used in a similar `_default` method for the `click` event,
is to check for the `dataPriv` entry on the element for the focus event
(similarly for blur).

Fixes jquerygh-4867
Closes jquerygh-4885

(cherry picked from commit e539bac)