Sortable module - zIndex of 999 is kept on dragged item

### Hello!

In our project we came across this bug (or maybe is it a feature?) :

* If we drag an element from a ```sortable``` list, the last dragged item keeps a z-index property of ```999```.
* Then if we later have a panel shown on top of the page (with a lower z-index of 999) the last dragged item will be in front of the given panel, which is clearly not wanted.

Here is then a PR containing a potential fix. Please give me a feedback whether this is acceptable or not.

Cheers,
Paul