The long-term goal is to use nose2 for testing

Changing "adding unsupported files" to "deleting unsupported files".
UX people told me that it's more clear this way.

This should close https://labs.riseup.net/code/issues/10839

Thanks to intrigeri for the idea

This should close https://labs.riseup.net/code/issues/10838

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.