Panel: Do not close in response to a default-prevented swipe event

Fixes gh-6925

Note: This fix requires https://github.com/jquery/jquery-mobile/pull/6946 even though the automated tests pass, because in the automated tests I trigger swipe manually, with .trigger( "swipeleft" ) instead of calculating it from mouse events using the swipe special event. https://github.com/jquery/jquery-mobile/pull/6946 ensures that the swipe special event triggers correctly.
This whole `if...else` can be simplified to
```
var event = {};
    event[ "swipe" + this.options.position ] = "_handleSwipe";

this._on( area, obj );
```