Add support for GTK 4
These changes let reflex-gi-gtk be used with GTK 4.
Library backwards compatibility with GTK 3 is achieved with a little CPP. For GTK 4, the old box functions gtk_box_pack_start and friends are emulated.
These changes let reflex-gi-gtk be used with GTK 4.
Library backwards compatibility with GTK 3 is achieved with a little CPP. For GTK 4, the old box functions gtk_box_pack_start and friends are emulated.