Renamed preparePlainTextValue

This also adds two new methods on the field class:
1. `prepareReadableValue` which must return a readable string,
optionally filtered or process for better readability.

2. `prepareTextValue` which must return a complete string representation
of the field's value(s).

It also refactors a bunch of file in order to replace
`preparePlainTextValue` calls to `prepareReadableValue`.
Fields that were overriding `preparePlainTextValue` now implements
 `prepareTextValue`