Make MetaRectangle a boxed type

Export a boxed type for MetaRectangle; this is mostly of interest
because GdkRectangle has been turned into a typedef for
cairo_rectangle_int_t which causes massive problems for using it from
gobject-introspection based language bindings.

https://bugzilla.gnome.org/show_bug.cgi?id=623335

src/include/boxes.h

@@ -24,9 +24,11 @@
 #ifndef META_BOXES_H
 #define META_BOXES_H
 
-#include <glib.h>
+#include <glib-object.h>
 #include "common.h"
 
+#define META_TYPE_RECTANGLE            (meta_rectangle_get_type ())
+
 typedef struct _MetaRectangle MetaRectangle;
 struct _MetaRectangle
 {
@@ -70,6 +72,11 @@ struct _MetaEdge
   MetaEdgeType  edge_type;
 };
 
+GType meta_rectangle_get_type (void);
+
+MetaRectangle *meta_rectangle_copy (const MetaRectangle *rect);
+void           meta_rectangle_free (MetaRectangle       *rect);
+
 /* Output functions -- note that the output buffer had better be big enough:
  *   rect_to_string:   RECT_LENGTH
  *   region_to_string: (RECT_LENGTH+strlen(separator_string)) *