fix to Atril stopped opening cbr and cbz comic documents when upgraded to GTK 3.22

[Àlex]

This last month my Debian Testing upgraded GTK 3.20 to GTK 3.22. Then
Atril 1.14 and Evince 3.21 stopped opening cbr and cbz files. Evince
fixed this problem in the upgrade to Evince 3.22 but Atril didn't fix
the problem in the upgrade to Atril 1.16.

It seems that the problem is something related to how GTK 3.22 works
with mime types. The comics backend is using string equality to match
mime-types. This doesn't work when the mime-type has changed, and the
old name is now simply an alias. Must use g_content_type_is_a() instead.

Atril developers have to change in
atril/backend/comics/comics-document.c some strcmp (mime_type, ...) with
g_content_type_is_a (mime_type, ...) in four places:

http://osdir.com/ml/commits.gnome/2016-09/msg04181.html
https://bugzilla.gnome.org/show_bug.cgi?id=770903

For normal user, there is a little workaround that temporaly fixes that,
just editing
/usr/lib/x86_64-linux-gnu/atril/3/backends/comicsdocument.atril-backend
and add at the end:

MimeType=application/x-cbr;application/x-cbz;application/x-cb7;application/x-cbt;application/x-ext-cbr;application/x-ext-cbz;application/x-ext-cb7;application/x-ext-cbt;application/zip

Excuse my poor English


Àlex