Remove container class bitfield
GtkContainerClass has a special hack using a bitfield. The gtk-rs gir crate cannot process bitfields yet, and so it cannot reconstruct the C class structs from .gir data that has bitfields.
This MR replaces that field with a gpointer, to preserve struct alignment, and hopefully gtk-rs's gir will now be able to process GtkContainerClass correctly
(This is needed to derive widgets in Rust.)