Improve tooltips

[beto-rodriguez]

Multiple improvements to tooltips.

Fixed issues

fixes #849
fixes #869
fixes #1046

Breaking changes

1. IChartTooltip.Hide() changed

This should only affect you if you built a custom tooltip, this change prevents a tooltip to hold a refence to the chart, instead LiveCharts will inject the chart.

IChartTooltip<SkiaSharpDrawingContext>.Hide() method changed to IChartTooltip<SkiaSharpDrawingContext>.Hide(Chart<TDrawingContext> chart)

This is an example of how to migrate a custom tooltip from version beta.710 and lower:

Previously

public class SKDefaultTooltip : IChartTooltip<SkiaSharpDrawingContext>
{
    private Chart<SkiaSharpDrawingContext> _chart;

   // then we used to set the _chart in the Show() method.

    public void Hide()
    {
        if (_chart is null || _panel is null) return;
        _chart.RemoveVisual(_panel);
    }
}

Now

public class SKDefaultTooltip : IChartTooltip<SkiaSharpDrawingContext>
{
    public void Hide(Chart<SkiaSharpDrawingContext> chart)
    {
        if (chart is null || _panel is null) return;
        chart.RemoveVisual(_panel);
    }
}

2. Some event handlers were simplified

For the series: LineSeries, StepLineSeries, StackedAreaSeries and StackedStepAreaSeries the ChartPointPointerDown, ChartPointPointerHover and ChartPointPointerHoverLost events are now much simpler, if you are using any of those events in any of those series, you must update the handler method. This makes all the series events consistent.

This is an example of how to migrate from beta.710 and lower:

Previously

var lineSeries = new LineSeries<int>();

lineSeries.ChartPointPointerDown +=
   (IChartView chart, ChartPoint<int, BezierPoint<CircleGeometry>, LabelGeometry>? point) =>
    {
        // point was clicked!
    };

Now

var lineSeries = new LineSeries<int>();

lineSeries.ChartPointPointerDown +=
    (IChartView chart, ChartPoint<int, CircleGeometry, LabelGeometry>? point) =>
    {
        // point was clicked!
    };