cell type category coloring conventions

[cornhundred]

Is your feature request related to a problem? Please describe.
We need to keep a consistent color scheme for cell types across different experiments.

Describe the solution you'd like
We want to be able to keep cell types and colors linked to each other. So we should have

• colors pre-defined for preferred categories that are brought up when a preferred category is selected
• we need to make a similar API for getting/setting category colors of non-manual categories
• colors should be saved as metadata for categories (other metadata will include parent category
• manual categories that are not part of the pre-defined set of categories should have their custom colors saved to the metadata and be able to update the globbal_category_colors

Describe alternatives you've considered
None

Additional context
None