Insert image(s) by pasting

As a user, I want to be able to insert an image into a draft by pasting the image.

See https://github.com/OpeningDesign/draw/issues/12 and #2 (comment 150939530)

Requirements:

• add paste event handler on element (the document?)
• insert image from pasted url
• doesn't create image if image not returned from url
• upload pasted image
• insert uploaded images to the image layer (behind the annotation and drawing layers (see #67 (closed))
• auto name increment to avoid name conflict (see #57 for replace/rename prompt)
• show pasting and uploading progress in status bar
• Error on the app before uploading when the image is above the maximum size
• show loading progress when loading images
• Hide image button for now
• Delete image when removed from draft