Datepicker: Hide the UI on destroy

Datepicker: Hide the UI on destroy

When the datepicker UI is shown and then destroyed programmatically:
```js
$( "#datepicker" ).datepicker( "destroy" );
```
hide the datepicker UI without the need for an explicit user action.
Previously, in 1.12 the UI would not disappear immediately but only after the
first `mousedown`. In later 1.13 versions, the UI would not disappear at all.

Fixes gh-2178
Closes gh-2268