Iterate on formatter code; add LinkCeFieldFormatter

This is minor, so I'm not going to finish it, but I'm already going to commit AND merge the changes made in a custom project, that should be moved into here:

• Make protected get**Value() methods inside FlattenedFieldFormatter (moving it into a trait) and RawCeFieldFormatter more easily extensible
• Drop protected RawCeFieldFormatter::processRaw() at the same time: I don't see how that one helps
• Move isApplicable() code into field types in annotations, where they belong
• Add LinkCeFieldFormatter.

Still need to do: create test for LinkCeFieldFormatter, and point 2 (image style config).