Allow to hook into translatebles discovery

Assuming that potx is still in l.d.o legacy mode according to #3344899-3: Offering to co-maintain Translation template extractor → .

So being super conservitive about the API for now, which should also make the patch trivial to review.
Added hooks for _potx_explore_dir, _potx_process_file, potx_finish_processing, with exactly same signatures.
Which should be enough-for-everyone (tm).

geek-merlin → closed merge request !19

Also adding a branch combining this with ✨ Make 'folder' option multiple RTBC . Can be ignored for the sake of this issue.