Save image attachments in the diagram source using their reference instead of their URL

[mflorea]

ATM when you insert / upload an image from your computer, the image is attached to the diagram page and when you save the diagram the diagram source contains the attachment URL. This makes it hard to refactor the diagram source when the attachment or the diagram page is renamed / moved. Moreover, when you edit the image from the diagram editor you see the attachment URL.

The goal of this issue is to:

• the source of the diagram should contain the attachment reference when the diagram is saved, and not the attachment URL
• when editing the attached image, the user should see the attachment reference, and they should be able to enter a different attachment reference.