Support multiple editor instances when drag-and-drop-uploading onto them.

Also reduces z-index to below the toolbar, and adds a comment.

props kovshenin.
see #19845.

Built from https://develop.svn.wordpress.org/trunk@27378


git-svn-id: http://core.svn.wordpress.org/trunk@27227 1a063a9b-81f0-0310-95a4-ce76da25c4cd