snap: add KDE's Breeze GTK theme

This adds KDE's Breeze theme, as described in bug #6 (closed). We probably also want to ship the corresponding icons, but I was having trouble getting that to build in the xenial container (it won't build with the older extra-cmake-modules: not sure if that's a hard requirement or the developers just ratchetting it up with the KDE version).

I dropped the meson part: with the gresources change in Yaru, Meson complains that xenial's glib-compile-resources is too old. If we can't build with snapcraft cleanbuild anyway, we may as well rely on meson from the Docker container our CI is running in.