display: Move the pointer event handling code to work in terms of Clutter events

There is now a meta_display_handle_event alongside the
meta_display_handle_xevent function which handles events in terms of
Clutter events instead of X events. A Clutter event filter is
registered so that all Clutter events will pass through this function.
The pointer event handling code from the X event version has been moved
into this new function and has been modified to use the details from
the Clutter event instead of the X event. This is a step towards
moving all of the event handling code over to use Clutter events.

Based-heavily-on-a-patch-by: Neil Roberts <neil@linux.intel.com>

src/core/display-private.h

@@ -39,6 +39,7 @@
 #include "keybindings-private.h"
 #include <meta/prefs.h>
 #include <meta/barrier.h>
+#include <clutter/clutter.h>
 
 #ifdef HAVE_STARTUP_NOTIFICATION
 #include <libsn/sn.h>
@@ -189,7 +190,7 @@ struct _MetaDisplay
   MetaWindow* autoraise_window;
 
   /* Alt+click button grabs */
-  unsigned int window_grab_modifiers;
+  ClutterModifierType window_grab_modifiers;
   
   /* current window operation */
   MetaGrabOp  grab_op;
@@ -485,6 +486,9 @@ guint meta_display_get_above_tab_keycode (MetaDisplay *display);
 gboolean meta_display_handle_xevent (MetaDisplay *display,
                                      XEvent      *event);
 
+gboolean meta_display_handle_event (MetaDisplay        *display,
+                                    const ClutterEvent *event);
+
 #ifdef HAVE_XI23
 gboolean meta_display_process_barrier_event (MetaDisplay *display,
                                              XIEvent     *event);