Override media fields from the reference field

Problem/Motivation

When reusing the same image in different places, the alt text of the image might be different for each use. Since images could contain a lot of elements, you might want to describe different parts of the image depending on the context of how the image is used. It is currently not possible to override the canonical alt text of a media image on a per use basis. (Similarly, different tags might be needed depending on where it is used.)

As the WCAG makes a distinction between:

• Informative Images - where the text alternative should convey the meaning or content that is displayed visually
• Decorative Images - where a null (empty) alt text should be provided (alt="")

This is also discussed in 🐛 According to the WCAG guidelines decorative images should not have an alt-text but this is not possible Active

Proposed resolution

Add an map field to the media entity reference to store the field overrides. Note the override is per fieldand not per field_property.

Remaining tasks

Add more doxygen to the media item class, especially around the per field vs per property override.

User interface changes

API changes

Data model changes

Release notes snippet