make sure it's OK to use DragHandler in a ListView delegate

Maybe there have to be some limitations, maybe not: if you drag something inside the delegate in the same direction as the Flickable can flick, what should happen?

We already have this use case in the mixer manual test: the sliders can drag vertically and the ListView can drag horizontally. That works OK. But some users might want it to somehow work even when you drag the DragHandler the same direction as the Flickable's enabled axis. Even if it has to involve some ugly imperative API like setting the accepted state of the event or whatever.