Pinch to zoom [WIP]

This PR aims to implement pinch to zoom functionality for mobile devices as described in #2582. Current;y, pinch to zoom is implemented, but it is not usable yet. Feedback and tips are appreciated!

- [ ] Zooming increases/decreases slowly. Start when pinching at scale 2.4 for example, but decreases to 1.1. We only need to get the last scale factor (as we also get all intermediate ones while pinching), but I have not yet found a way on how to do this.