Add a private function that lets us decide at runtime whether to keep css section information around - this will be useful in the GtkInspector.
... and document it. For now, the function is only exported internally.
This reverts commit 1f5dea9eba, since it was causeing noticable behaviour changes. Previously, GTK_DATA_PREFIX=/ ./gtk3-demo would start gtk3-demo with the Raleigh theme. With that change, it was starting with no theme at all (i.e. all black).
1f5dea9eba
https://bugzilla.gnome.org/show_bug.cgi?id=683896