Default App: gnome-sushi

[Jeremy Bicha]

Now that gnome-shell is in the default Ubuntu 17.10 daily image, I
think we could maybe start talking about other default apps. If we
want new stuff in main, I think it's good to start the Main Inclusion
process early.

First, how about gnome-sushi? (Upstream's name is just 'sushi').

Sushi is a file previewer for nautilus. It can be activated by
pressing the spacebar when a file is selected. Sushi has been a part
of core GNOME since GNOME 3.2. It is described in the default user
help bundled with GNOME. [1]

Sushi was never really considered for inclusion in Ubuntu's default
install earlier because it uses gjs which was not desired in Ubuntu
main until we needed GNOME Shell.

There is one universe dependency: libmusicbrainz5. An earlier version
of this library, libmusicbrainz3, was in main in Ubuntu 12.04 LTS.

[1] https://help.gnome.org/users/gnome-help/stable/files-preview.html
or you can run the installed version:
yelp help:gnome-help/files-preview

Thanks,
Jeremy Bicha