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)?
`_dispatchEvent(originalTarget, 'filter', target, undefined, startIndex);` ;]