Add dragSelection methods, ignore scrollbar clicks

Resolves #181 

While adding functionality to ignore clicks on the scrollbar, there was found to be two 'mousedown' listeners in the Drawer.  

To combine them into one 'mousedown' listener, I allowed 'mousemove' to always track a POSSIBLE drag selection, but not raise the event unless the feature is active.  

This then created the ability to modify the 'dragSelection' parameter via three new methods.


Great, thanks!