If the configured icon theme (e.g. Sweet-Blue) doesn't exist on disk,
GTK falls back to hicolor which has almost no icons. Detect this by
probing for a standard icon, and set Adwaita as the theme — it's
guaranteed to exist as a GTK4 dependency.
This replaces the broken add_search_path("/usr/share/icons/Adwaita")
approach which doesn't work because search paths are scoped to the
active theme name, not the directory name.
vikingowl
2b98f0651c