Remove draggable behaviour

[jwarby]

Remove the draggable behaviour feature, which allows user to drag the tooltip around. It's not used and seems a bit pointless.

The only use case I can see for this allowing the tooltip to be dragged on-screen in the event that it extends the edges of the screen. However, I also see that particular use-case as a workaround for the issue - in fact, a separate issue has been raised to address this potential problem (issue #39)