Allow to tweak opacity for each mesh individually

[philippotto]

Detailed Description

In addition to simply toggling the mesh visibility, it can be useful to also explicitly set the opacity of a mesh. Likely, we can just use RGBA instead of RGB values for the mesh colors to enable opacity.

Estimated Cost

2d?

Implementation

• opacity should be stored in the mesh object (which is not persisted currently; persisting the mesh items is a separate issue)
• update docs

[normanrz]

We could use the antd color picker, which has rgba support with an alpha slider.

[hotzenklotz]

Our current React color picker also has support for RGBA / alpha channel. See demo at: https://omgovich.github.io/react-colorful/