Start populating MetaX11Display

- Moved xdisplay, name and various atoms from MetaDisplay
- Moved xroot, screen_name, default_depth and default_xvisual
  from MetaScreen

- Moved some X11 specific functions from screen.c and display.c
  to meta-x11-display.c

https://bugzilla.gnome.org/show_bug.cgi?id=759538

src/core/restart.c

@@ -43,6 +43,7 @@
 #include "ui.h"
 #include "util-private.h"
 #include "display-private.h"
+#include "x11/meta-x11-display-private.h"
 
 static gboolean restart_helper_started = FALSE;
 static gboolean restart_message_shown = FALSE;
@@ -191,7 +192,8 @@ meta_restart_finish (void)
 {
   if (is_restart)
     {
-      Display *xdisplay = meta_display_get_xdisplay (meta_get_display ());
+      MetaDisplay *display = meta_get_display ();
+      Display *xdisplay = meta_x11_display_get_xdisplay (display->x11_display);
       Atom atom_restart_helper = XInternAtom (xdisplay, "_MUTTER_RESTART_HELPER", False);
       XSetSelectionOwner (xdisplay, atom_restart_helper, None, CurrentTime);
     }