Add new property to QAbstractSeries to allow hiding a series in the legend of a QChartView

The legend for a chart view currently is an all-or-nothing deal, it is not possible to specify on a per-series basis whether a series should be visible in the legend.

There are uses cases (e.g., showing a fit through data) where certain series should not be visible in the legend - adding an additional 'visibleInLegend' property to the QAbstractSeries would add flexibility that is currently missing in QML.