Add option to display legend for chart item

[DelazJ]

Feature description

1. In a print layout, you can add a chart item of different types.
2. You can add series of data to plot, which render nicely.
3. But in the chart item options there is no way to label what a series actually represents. Series title can e.g. be set in the "Series details" panel.
   A way to display a title for the chart, as part of the item options, would also be great.

Additional context

No response