img attributes to facilitate the use of Responsive Images & Media Libraries

[aSeriousDeveloper]

For #249
Recreation of #489

• src / size are for responsive image support a la Spatie Media Library
• data-media-id is for storing / parsing IDs used by a Media Library of any sort

The reason I recreated this was due to inactivity on the other PR, and to split the media item changes into a PR separate from adding a new iFrame node.

The iFrame node would still be beneficial, in my opinion, but I'll let @rubenlopezgea-at-coodex manage that.

[awcodes]

Where would the srcset and sizes be defined?

[aSeriousDeveloper]

Where would the srcset and sizes be defined?

These would be provided by the MediaAction and EditMediaAction, however I didn't add these to the core versions of these actions as I felt putting these attributes into the defaults of these actions would be out of scope / confusing for people using them, instead allowing them to be exposed by other packages that may provide these in other ways.

The most notable example being Spatie's media library which generates responsive images, as well as any packages which extend its functionality like Ralph J Smit's Media Library Manager.

I can look at modify this PR to have the default actions potentially provide an option to fill in the srcset and sizes attributes, although I'm not sure how that implementation would look. Probably inputting as key/values, parsing that as a string, then doing the reverse for edit?

[awcodes]

I'm ok with them not being default. Prefer it that way. Should the Readme cover the usage though?

Also need to make sure it's doesn't break anything with Curator.

[aSeriousDeveloper]

I can update the readme to mention the new attributes.

[aSeriousDeveloper]

Readme updated to give a quick explanation of the new attributes.