Icon theme not present
Context
The freedesktop SDK is expected to provide a base icon theme. 18.08 doesn't appear to, which is a regression on 1.6 and makes file open dialogs look really nasty.
Acceptance Criteria
Compare:
flatpak run --command=gtk3-widget-factory --socket=x11 org.freedesktop.Sdk//18.08
With:
flatpak run --command=gtk3-widget-factory --socket=x11 org.freedesktop.Sdk//1.6
They should render the same (including icons).