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.
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manual #148320
win32 allowed to fail manual
msys2-mingw32