Add alt-tag field to image content type

[jackahl]

Currently in many cases in Volto there is no alt text displayed for images(like plone/volto#6302), cause there is no real way to add a proper alt text. We could use the description field as suggested in #605, but this should probably be its own field, as they serve different purposes in some cases.

[jackahl]

So I had a chat with @MrTango and he agrees that there would be indeed use for this in Classic as well. We should just make it very clear in the field description that this field is only for use in automated cases like listings/collections and still force people to add an alt tag manually when using the image explicitly. I would like to hear some opinions on this @sneridagh @tisto @davisagli @JeffersonBledsoe

[jackahl]

I also noticed part of this was also addressed 5 years ago in #518 :D

[yurj]

Actually, in Classic-UI, Plone fill the title attribute using the alt value. I think this is wrong, title should not be filled in images which uses the alt attribute. For other content or links, it is ok to use title.

[thet]

Related: plone/plone.namedfile#162