Before this PR, calling ChartThresholdPane.addThreshold() was adding a threshold line twice, because:

• addThreshold(final ObservableNumberValue d, final Color color) was adding a first line.
• At the end of addThreshold(final ObservableNumberValue d, final Color color), thresholds.add(d); is called. This triggers handleLineListChange() to be called.
• handleLineListChange() calls addThreshold(ObservableNumberValue), which calls addThreshold(final ObservableNumberValue d, final Color color): this adds the second line.

This PR fixes that by only creating the line in handleLineListChange(). The addThreshold() functions now just update thresholds (which triggers handleLineListChange()).