GdkWin32 Cleanup

TODO:

• apply patches by @otte from https://gitlab.gnome.org/GNOME/gtk/-/tree/wip/otte/win32-cleanup
• Move GdkPopup class code to gdkpopup-win32.c, GdkToplevel class code to gdktoplevel-win32.c
• From IRC:

really, the HWNDs should be created in custom ::constructed() functions for the different subclasses instead of the generic win32_surface_constructed