Add an option for a progressive node color scheme (gradient from top to bottom nodes)

[lilliputten]

It's required to add application setting for node colors options:

• "Random": based on node identifier values.
• Single color for the nodes (user selects one color).
• Progressive gradient mode (user selects two colors).

In all modes, user should be able to manually override certain node colors.

[lilliputten]

Finished in PR #11.