snap: add gtk-2-themes content interface slot

After discussion with didrocks on IRC, it seems the best way forward to support GTK 2 themes is to package the theme data (but not the engines) in the gtk-common-themes snap. Most of the theme upstreams ship both together, and this ensures that the two themes are in sync.

The architecture dependent gtk2-common-themes snap would then be restricted to engines, and perhaps should be renamed to gtk-2-engines or similar.