Tags: add case for a `list` of files' tags

This splits the type `Tags.t` into `Tags.file_format` and `Tags.t`
using sub-typing and propagates the change (hence this adds some
`not_implemented` error cases to the application).
Tags: add sub-module `Output_transform` (#102)

Most of the code is imported from `App_common`.
App_random: use `Tags.Output_transform` (#102)
App_transform: use `Tags.Output_transform` (#102)
App_common: remove obsolete output-transform code

This is related to the issue #102.