Rename gdk_window_set_has_native to gdk_window_ensure_native

Rename gdk_window_set_has_native to gdk_window_ensure_native

This also removes the (unimplemented) possibility to change a window to
non-native. This seems generally not very useful, and there are some problems
with it, for instance if two "users" need a window to be native and then
one of the "users" doesn't need it anymore it can't change it back, because
it is unaware of the other reason the window is native.