Support colorBy feature for marker border color also.

Currently colorBy method in QScatterSeries colors only the inner filled part of the marker. We should provide support to color border of the markers also according to color by property.

This will be useful when users are using hollow shapes (ie calling QScatterSeries::setBrush(Qt::NoBrush)) and making a call to colorBy method, expecting marker border colors to be colored according to the passed values.