Revert "Move MetaGroup to be X11-specific"

This reverts commit f940292cfe.

Turns out MetaGroup was public API and was used by gnome-shell.
Ugh.

src/core/stack.c

@@ -30,12 +30,13 @@
 #include "window-private.h"
 #include <meta/errors.h>
 #include "frame.h"
+#include <meta/group.h>
 #include <meta/prefs.h>
 #include <meta/workspace.h>
 
 #include <X11/Xatom.h>
 
-#include "x11/group.h"
+#include "x11/group-private.h"
 
 #define WINDOW_HAS_TRANSIENT_TYPE(w)                    \
           (w->type == META_WINDOW_DIALOG ||             \

src/core/window-private.h

@@ -44,7 +44,7 @@
 #include <clutter/clutter.h>
 
 #include "x11/iconcache.h"
-#include "x11/group.h"
+#include "x11/group-private.h"
 
 #include "wayland/meta-wayland-types.h"
 

src/core/window.c

@@ -39,6 +39,7 @@
 #include "ui.h"
 #include "place.h"
 #include <meta/prefs.h>
+#include <meta/group.h>
 #include "resizepopup.h"
 #include "constraints.h"
 #include "mutter-enum-types.h"
@@ -58,7 +59,6 @@
 #include "x11/window-x11.h"
 #include "x11/window-props.h"
 #include "x11/xprops.h"
-#include "x11/group.h"
 #include "x11/session.h"
 
 #include "wayland/meta-wayland-private.h"