implement pinch-to-zoom for touch screens

[tomcl]

Describe the solution you'd like It would be good to have gesture-based two finger zoom in addition to existing one-finger pan.

Describe alternatives you've considered Current key-based zoom is less convenient, as is "zoom to mouse rectangle"

Additional context npm has an easy to use (I expect) package https://www.npmjs.com/package/pinch-zoom-js

Or, a complete implementation based on pointer events would be possible, but a lot more work.

[adidesh20]

Pinch to zoom support would also be useful for laptop touchpads as it is more intuitive than Ctrl+Two finger scroll