Allow users to enter a long description for complex images

Image alt text has a limit of 125 characters. This is an issue when teachers need to provide alternative text for complex images such a charts and other learning material.

Whilst there are valid reasons to keep the alt text short, users are left with adding a description (or link to a description) as their only option. Technical skilled users may be able to go as far as adding custom html.

It would be great if there was a dedicate field in the image dialog that can be used so that the long description is added consistently and is output using the correct semantic markup. The description will also be available to all users and improve their leaning experience.

This issue was initially described in MDL-77044 for the Atto plugin