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)?
Should I replace

    _dispatchEvent(el, 'filter', originalTarget, undefined, startIndex);

with

    _dispatchEvent(originalTarget, 'filter', originalTarget, undefined, startIndex);

?