Determine the correct target before onFilter

Fix #156 and also returning correct indexes if a drag handle is present (#154).

How does this impact the [example code for `closest`](https://github.com/RubaXa/Sortable/blob/master/README.md#closestelstring-selectorhtmlelementhtmlelementnull)?
Is bad, `onFilter` should to work with `evt.target`.
Add variable `originalTarget`:
```js
originalTarget = (touch || evt).target,
target = originalTarget

// ...

_dispatchEvent(el, 'filter', originalTarget, undefined, startIndex);
```