Any tutorials or examples on the usage of GUI API?

[GCChen97]

Checklist

• [X] I have searched for similar issues.
• [X] For Python issues, I have tested with the latest development wheel.
• [X] I have checked the release documentation and the latest documentation (for master branch).

My Question

I really like Open3D's modern features and want to build my project based on Open3D. But the docs have no tutorials on how to write a customized window. I want to add some buttons and slides on the screen so that I can use them to set hyper-parameters for some algorithms.

Although there are some examples, they all use "pre-defined" visualizers, making it hard to learn how the GUI system of Open3D works. Things like how to add layouts or buttions or callback functions are missing in the docs.

I also really hope that Open3D can provide some API as does Pangolin. Because using Pangolin, it is very easy and straightforward to add buttons and slides for algorithm tuning, while in Open3D, doing such seems a bit tedious.