Decouple progress bar width and touchable area: add minimumTouchWidth argument

[tony-ditchlabs]

Use case:
When the user taps near, but not directly on, the progress bar, the widget doesn't update.

The problem:
In the _onPanDown and _handlePan methods, the touchWidth is defined as the greater of 25.0 or the progress bar width

The fix:
Change the number 25.0 to an argument in the constructor, minimumTouchWidth. This will let the designer/developer decide the fault tolerance for the slider. Setting it to double.infinity will ensure that every gesture is captured.