Added .fromItem constructor to BarChart

[matheus-kirchesch]

I did this to allow me to use the CharItem value from the data: property in the BarChart, example:

return BarChart.fromItem(
      data: [
        ChartItem(32.52, value: {
          'title': 'De 1 á\n16 dias',
          'value': '32.52',
        }),
        ChartItem(15.8, value: {
          'title': 'De 16 á\n30 dias',
          'value': '15.8',
        }),
        ChartItem(15.97, value: {
          'title': 'De 31 á\n60 dias',
          'value': '15.97',
        }),
      ],
      itemOptions: WidgetItemOptions(
        widgetItemBuilder: (data) {
          print(data.item);

          return Container(
            decoration: BoxDecoration(
              color: Theme.of(context).colorScheme.primary,
            ),
            margin: EdgeInsets.symmetric(horizontal: 4.0),
            child: RotatedBox(
              quarterTurns: 1,
              child: Align(
                alignment: Alignment.centerLeft,
                child: Padding(
                  padding: const EdgeInsets.symmetric(
                    vertical: 2.0,
                    horizontal: 8.0,
                  ),
                  child: Text('${data.item.max}%'),
                ),
              ),
            ),
          );
        },
      ),
    );

In this example I can use my custom property 'title' to dynamically draw it while rendering the bar.

Without this the only thing I have from my ChartItem is the max property, and it also makes the API simpler.

[matheus-kirchesch]

In the commit I also included the change from BarValue to ChartValue since BarValue has been deprecated, since it was a small change I decided to include in the same pull request.