Plain text value

As discussed here: efb694f381fef36545bcdccb8db5a24e1220b450

This PR looks big, but all commits are smallish and well documented.

@nilshoerrmann Should I really add coupling between Field and ExportableField ? Right now, Field is not coupled with ExportableField and I feel like the default implementation is enough. But it would prevent Field from having to overwrite both `preparePlainTextValue` and `prepareExportValue` instead of `prepareExportValue`only. I will gladly add it if you want to.

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ba3ade6 could be part of another PR, be ignored or be cherry-picked...