Bug#823714: gtk3-engines-breeze: add gtk3 (=3.18) to depends

[Jeremy Bicha <jbicha@ubuntu.com>]

I can confirm that gtk3-engines-breeze 5.6.95-1 works with GTK 3.20
but not with GTK 3.18 (as used in Ubuntu 16.04 LTS).

On the other hand, I don't see that any of the other GTK 3 themes in
Debian depend (or break) on GTK 3 at all.

It's possible for a GTK 3 theme to support multiple versions by using
directories like /usr/share/themes/gtk-3.20/ , gtk-3.18/ , gtk-3.0/ ,
etc. That's not very important for Debian which doesn't support
multiple GTK3 versions in the same Debian release but it could be
useful for a theme developer.

Thanks,
Jeremy Bicha