fix issue with position offset when the always show tooltip enabled is true

[NicosNicolaou16]

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Fixed an issue with tool tip position offset when the alwaysShowTooltip is true and set direction with FlutterSliderTooltipDirection.top (optional), so the tool tip is not showing or the handler overlap the tool tip

The fix is in this file: lib/src/view/flutter_slider_widget.dart

You can try the "master" branch with this code to see the issue and also you can see the issue below from print screens

Container(
         // width: 100,
         // height: 50,
         padding: EdgeInsets.all(50),
             child: FlutterSlider(
                values: [0],
                max: 100,
                min: 0,
                tooltip: FlutterSliderTooltip(
                    alwaysShowTooltip: true,
                    direction: FlutterSliderTooltipDirection.top,
                    //positionOffset: FlutterSliderTooltipPositionOffset(top: -25) //added this line to fix it temporary 
              ),
           ),
        );

[NicosNicolaou16]

Hello 👋 can you review my pull request please? 😁🙏@Ali-Azmoud