Mouse sensitivity adjustment

**Is your feature request related to a problem? Please describe.**
I want to adjust camera perspective (position, angle, zoom) using mouse in very small increments. Currently any mouse action (drag or wheel scroll) makes a big change to the camera, so the rendering appears jumpy and I couldn't get to a perspective that I need. Is there an existing feature to do this?

**Describe the solution you'd like**
If there is no existing feature to do this, can we consider this as a feature request?

**Describe alternatives you've considered**
No alternatives I could find.

**Additional context**
I'm using Open3D 0.11.2 on Ubuntu 18.