onTap from SensibleDev

This version fixes problems with gesture detection and adds onTap support. That means that the widget will behave differently with a drag and a tap. That way tapping doesn't need to automatically seek to some location.

See discussion at PR #49 from @thesensibledev

We aren't currently going to publish this version to Pub, but I'd like to keep the code for future reference (or for someone to use directly in their project).