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.


git-svn-id: https://develop.svn.wordpress.org/trunk@27378 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/edit.css

@@ -594,6 +594,9 @@ span.wp-media-buttons-icon:before {
 
 .edit-form-section {
  	margin-bottom: 20px;
+}
+
+.wp-editor-wrap {
 	position: relative;
 }
 
@@ -604,7 +607,7 @@ span.wp-media-buttons-icon:before {
 	left: 0;
 	width: 100%;
 	height: 100%;
-	z-index: 250000;
+	z-index: 99998; /* under the toolbar */
 	display: none;
 	text-align: center;
 }