Allow specifying overlay colors using css color names

[almarklein]

There are two parts where we deal with color:

• The slicers color property, used for the indicators. Supports anything compatible with CSS.
• The colormap for overlays. Currently supports RGB tuples, RGBA tuples, and hex colors.

[almarklein]

This is just a matter of having a dictionary that maps color names to hex colors. We can take such a dictionary from e.g. Matplotlib. However, if such a table already exists in Plotly or Dash, that would be much easier.