finish porting the Photo Surface example to Pointer Handlers

This work is started, but I want to add a global pinch-zoom feature to the app: if you pinch a photo you change its size, whereas if you pinch the underlying canvas you zoom the whole scene; and that turns out to be a bit tricky.

It's debateable whether to continue using Flickable (to show users that it's OK to mix them) or to use the opportunity to make sure FakeFlickable will work nicely in this use case.