GDK/Win32: Fix Windows backend after GdkWindow simplification

GDK/Win32: Fix Windows backend after GdkWindow simplification

Fix the build after the branch wip/alexl/simplify-gdkwindow was merged, as
there are some changes that broke things in the Windows backend, namely:

-gdk_win32_input_shape_combine_region() should not be removed at this
 point (though it is a stub--otherwise GDK/Win32 will crash)

-Some more code need to be removed due to the removal of items in the
 above-mentioned merged branch

Also, like the X11 backend, do not allow the creation of native child
windows, and stop checking for subsequent child windows
(GDK_WINDOW_CHILD), so that we can clean things up a bit.

https://bugzilla.gnome.org/show_bug.cgi?id=773299