Changed to work without ColorAxis.

[SADASHIMA]

• Add Fill in PolygonSeries class
• polyColor works as follows
  • If Fill Is Undefined
    • If Is ColorAxis valid
      • Get color from ColorAxis
    • Else
      • Get Default Color
  • Else
    • If Fill Is Automatic
      • Get Default Color
    • Else
      • Fill
• Added an example without ColorAixs

[SADASHIMA]

If the usage is the same when using ColorAxis and Value and when using Fill, I think the Fill should be defined in PolygonItem.
In that case, I also think the Stroke should be defined in PolygonItem.
What do you think about this?

[VisualMelon]

Yes, I'd be in favour of allowing full control of the presentation in the individual items

Thanks for working on this, I'll try to review your PR properly over the weekend