Fix for flickering issue when using a drop overlay

I was having problems with flickering (as a few of the other pull requests also call out) when using a translucent overlay div that would cover the dropzone. As soon as the overlay would come out, the dragleave event would fire on the drop zone, and flickering would ensue. What I did was introduce a new element nv-file-drop-curtain that parallels nv-file-over, except it expects to be an overlay over the entire drop area. When in play, the curtain is responsible for for the dragleave event rather than the drop zone.

I haven't tested this with multiple drop areas on a page.