Comment 4 for bug 1698795

For now, let's use in the ubuntu session non symbolic icons:
- our theme doesn't support them and there are few changes we'll have app symbolic icons for our default applications in 17.10.
- for coherence reasons: most of branded apps outside of GNOME (firefox, chrome, thunderbird) don't have symbolic icons, and so, will appear colorized. Shipping two different experiences in the same session is weird. Let's ensure the ubuntu session don't use them by default thus.