While doing this cleanup, I found at least several other string leaks
in: file-compressor, file-gegl, file-pdf-save, file-raw-data, file-xwd,
jpeg-load, psd-save…
So it's quite worth it!

Note: in file-pdf-save, there is a global variable file_name which seems
to be happily leaked without caring (didn't look in details, but looks
so). I didn't fix this one which will require a bit more in-depth logics
care.