XWIKI-19961: Copy/paste of HTML should allow copying images

XWIKI-22611: Copy and pasting images using the browser's "Copy Image" option does not upload an attachment to the wiki but adds an external URL image
* Download the images from the pasted HTML (or use the image files from the clipboard) and upload them after the HTML is inserted
* Use a CKEditor notification to indicate the progress and to give the user 5 seconds to cancel the upload
* Add a functional test for the HTML paste (unfortunately it's not possible to simulate the Copy Image option from the browser's context menu)