icontheme: make has_(g)icon consistent with lookup
Make gtk_icon_theme_has_icon
and gtk_icon_theme_has_gicon
also
consider unthemed icons. This makes their behavior consistent with the
actual (documented) lookup behavior.
Fixes: #5709 (closed) and makes the workaround in nautilus@b643a00b obsolete
See nautilus#2789 (closed) for further discussion.