Add showOnTopOfTheChartBoxArea for BarTouchTooltipData

Hello, the fl_chart team. 
Please add showOnTopOfTheChartBoxArea for tooltip BarTouchTooltipData. At the moment, tooltip has TooltipDirection.top and TooltipDirection.bottom. But there is no way to always draw a tooltip over a chart, as in LineChart.

May be add new flag like showOnTopOfTheChartBoxArea, or new TooltipDirection like TooltipDirection.over

Thank you!