Where possible, instantly preview images being uploaded

Problem:
When you:

• Paste an image in the editor or
• use the insert > image dialog in the editor to upload an image
• Drag and drop a single image into the editor

it:

• can sometimes feel slow
• doesn't give you a preview of what is uploading in-place (if you are doing it in the editor)

Suggestion:

• If you CMD+v or drag and drop a single image in the editor (if the browser supports it) we don't show the modal upload dialog, but so it instantly in place with a progress bar on it.
• If you use the image dialog, we do the same, but show the progress bar overlaid on top of the image as it's uploading, and you can't hit "done" until it uploads.

Things to think about:

• What do we do about multiple files being uploaded at the same time (through drag/drop) - in that case, we just fall back to the modal dialog.
• What do we do about error message reporting?
• Fallback for unsupported browsers