Make gdk_window_{lookup,foreign_new}_for_display backend specific
At the same time, make GDK_IS_DISPLAY_X11 available in gdkx.h, and add some exemplaric ifdefs to GTK+ code.
At the same time, make GDK_IS_DISPLAY_X11 available in gdkx.h, and add some exemplaric ifdefs to GTK+ code.