QtCharts QXYSeries: Add custom legend markers

Feature request to add custom legend markers for XYChart lightMarkers (which is itself a new feature requested in QTBUG-92884).

In the legend, show markers if markers are used. Scatter plot already has this behavior, but with the addition of light markers, light markers should also be displayed in the legend. This requests that feature.

This is implemented already, and it is just waiting for review.