Introduce pinch to zoom in camera view

This is working for the simple case, with a few caveats:

- No UI showing the current zoom scale
- Returning to camera view forces a zoom reset to 1.0

Ref #9
Add missing constraints to new preview view

Fixes layout issue when rotating camera
Add custom slider to track zoom scale
Merge pull request #85 from nova-device/feature/pinch-zoom-camera-9

Pinch & Zoom