use Pointer Handlers to replace or re-implement Flickable somehow

We have the FakeFlickable manual test, but of course it omits some features so far, and we probably don't really want to write it all in QML anyway.

Changing the Flickable class itself will be hard because it's inherited so much. So we can either reimplement it internally, using the Pointer Handlers C++ API, or offer a replacement for optional use.