icons: Set is_symbolic in IconPaintable.new_for_file

Otherwise symbolic icons won't be recognized as such.

Currently, in apps like Icon Library / App Icon Preview. Trying to render the generated on-fly symbolic icons, require caching them in a directory that mimics an icon theme and updating the search path of the default gtk::IconTheme. That is mostly because Gtk.IconPaintable.new_for_file wouldn't set is-symbolic even if the passed file is a symbolic icon.

This would allow us to remove all the hacks in our apps