Improve default image alternative to potentially use the attachment description field

Attachmment have a comment field that would be used to provide a description of the attachment. In most cases this would end up being a better default alternative for images inserted in content using this attachment than what we have currently (which is just "Image"...).

This is not a perfect image alternative text, because alternative text is always context dependant, while an attachment is context independant.

See https://jira.xwiki.org/browse/XWIKI-21938 and https://forum.xwiki.org/t/create-a-description-field-for-page-attachments/14346/5 for info and updates about this attachment description field

Note that this field should not be used if left empty (fallback would be "Image" that we had before).