GtkContainerClass._handle_border_width - replace bitfield with a gpointer
The gir processor from gtk-rs doesn't handle bitfields yet, and the presence of that field in GtkContainerClass prevents it from reconstructing the C struct out of the .gir information.
This commit replaces that bitfield with a gpointer, to preserve struct alignment. The generated .gir will be usable by the Rust crate.