Fix some visual glitches and problems using Drag and Drop.

When dragging a file between the "Drop Screenshot" and "Drop File Attachment"
boxes, the drop screen would flicker on and off. The previous code attempted
to prevent this by keeping a counter for dragenters/leaves, but it didn't
really work.

This also appeared to have prevented drag and drop from working on Firefox.

Now we delay hiding the screen using a timer. On dragenter, we just clear
the timer. It's now smooth going between the boxes, and I can upload files
on Firefox again.

It also does some additional cleanup, renaming things, updating docs, and
removing some excess variables.

Dragged files around the browser window. It was smooth and not flickery.
Uploading as screenshots or files worked.