adapt swtchart piechart slice color to dark/light theme

[mierin12]

previously slices were white in any theme

Hello, this is a proposition to adapt the color of the slice of swtpiechart to the black/white them. The HTML pie charts do adapt their slice color already.

Before :

After :

also applicable to the pie chart widgets.

HTML pie chart : already adapting the slice color

One little downside : I think it is a bit more noticeable when color sticks out of the pie, "toward the inside". Well, in white it was the opposite : noticeable "toward the outside", while no longer as much noticeable in black.

In white theme, I do not notice any change in behavior.