Media: Use field mapping to automatically store metadata/EXIF info

Part of #378; copied from https://github.com/backdrop/backdrop-issues/issues/2887#issuecomment-405...

I was playing around with D8 core and contrib media-related modules, and this specific feature drew my attention:

...so, media types have this "Field mapping" feature (the screenshot above is for the "Image" media type), which allows you to store metadata information to entity fields. I know we are a long way from anything like that (or are we?), and I don't wanna derail this issue here, but I thought that we could perhaps be using this to be automatically storing EXIF data to such a field (that we could be hiding by default). This could help us retain the EXIF data across any edit operations I believe.

Just a thought.