Fixes #1280, tray icons not drawing background. This is a magic pattern only
usable for gdk_window_set_background_pattern() that sets the underlying
X window's background to ParentRelative.

We don't support multiple screens anymore, so there is no need
for marking API as multihead safe any longer.

This is in preparation to modernizing our handing
of exported symbols.

gcc has optimizations for include guards that only work
if they are outermost in the the header.
https://bugzilla.gnome.org/show_bug.cgi?id=689810

Previously people used GDK_GPOINTER_TO_NATIVE_WINDOW() for this, but
native windows are going away, so we need a replacement.

Benjamin Otte authored Dec 20, 2010 and Matthias Clasen committed Dec 21, 2010

Now gdkx.h is a clean header equivalent to gdk.h.