Multiple changes to reduce the amount of unprefixed symbols

The X11 backend exports a number of symbols which are _-prefixed
(so don't become part of the gdk api), but are not named in a
way to prevent accidental clashes between backends.

The one API change here is that the gdk_xid_table functions
have been removed - they did not server an purpose, since the
xid table only stores windows anyway, and we already have a
lookup-by-xid function for windows.

gdk/x11/gdkvisual-x11.c

@@ -101,7 +101,7 @@ gdk_visual_x11_class_init (GdkVisualX11Class *class)
 }
 
 void
-_gdk_x11_visual_init (GdkScreen *screen)
+_gdk_x11_screen_init_visuals (GdkScreen *screen)
 {
   static const gint possible_depths[8] = { 32, 30, 24, 16, 15, 8, 4, 1 };
   static const GdkVisualType possible_types[6] =