Convert SeekerTail into a Drawable

[QriusDev]

Action Items:

• Create a class called TimelineSeekerTail that extends Drawable, in a new file /widgets/qp_timeline_seeker_tail.js
• Add this script to index.html, somewhere above <script src="widgets/qp_timeline.js"></script>
• Add property to the Timeline class, seekerTail = new TimelineSeekerTail(this.canvas)
• In the drawFunction override of the Timeline class, replace the old draw code with the set up code of the new and improved seekerTail.
• Since TimelineSeekerTail is just a glorified rectangle, we won't override the drawFunction for it. We'll just use the default.

[QriusDev]

This is blocked by #3 Please ignore until that issue is approved.

[QriusDev]

Its approved