This should fix https://labs.riseup.net/code/issues/10065

There is _no_ image support without exiftool, and _no_ audio support
without mutagen. Not "limited" support.

I forgot to add `gi.require_version('Poppler', '0.18')` everywhere,
and `gi.require_version('Gtk', '3.0')` in mat-gui too

Closes https://labs.riseup.net/code/issues/10550
Also known as "pygi is emitting a lot of warnings"

Now, the popup to add files to the interface
only shows supported files by default. It's of course
still possible to chose to show every possible files,
but it's not the default behaviour anymore.

a027c0b9 was a mistake.

It seems that Python is still sucking hard when it comes to encoding.

Namely, file-format specific ones

`mat` will now show help message

In the past, this might have been useful, but now,
exiftool works on pretty much every file passed to it by MAT.

Also, if something goes wrong, MAT will know about this thanks to the
return code != 0.

- Document some methods and the class
- Simplify a condition

This commit makes metadata removal of torrents recursive.
It's close to useless, because I haven't seen any torrent with a
recursive structure, but the documentation says that it can happen, and
the method was already implemented, so better safe than sorry.

This (huge) commit removes completely hachoir from MAT.
Audio files are now processed with mutagen, and images
with exiftool, since the main python imaging library (PIL)
isn't super-great to deal with metadata (and damaged/non-standard
files).

Package maintainer should change the dependencies to reflect this.

- PIL is not used anymore
- The testsuite should be launched with `python test.py test`