Add native pinch gesture support in PageView
This is necessary to support not just touch gestures but also touchpad gestures. Apparently QGesture is not implemented for Wayland, but QNativeGesture is...
There is a small polish issue where zooming and panning cannot be done in the same movement (like it can in Firefox), but Gwenview has the same limitation and I dont think anyone ever complained about it there.
Furthermore, on my machine, zooming causes quite a bit of flickering, but this also happens with ctrl+scroll when using hi-res scrolling. So that's a separate issue.
For reference: in Gwenview, it is handled at an even lower level (the Wayland protocol, gwenview!171 (merged)), but QNativeGesture seems to also work now and prevent some low-level plumbing. Perhaps Gwenview should implement the same.