Add way for users to specify color of nodes

[pogi7]

Checklist before submitting a feature request

• [x] I have reviewed CONTRIBUTING.md document
• [x] I have done a quick search on the issue tracker to check if the feature request is not redundant

Is your feature request related to a problem? If yes, describe the problem

It's annoying that the

colorKey

in the diagramLayouts.json does not allow users to specify the color of nodes.

An example of a node is specified below:

Describe the desired feature

Allow users to specify the color of node. Preferably in a GUI

Additional context

[Any additional context]

[pogi7]

Targeted for v0.2.0 release

[UTNAK]

In addition to specifying the box color manually, It's nice to have automatic coloring function using colorKey similar to defining color of edges.