Extend LdDiagramObject.

* Add generic methods for data storage, making it able
  to store data in a hierarchy.
* Reimplement ld_diagram_{get,set}_data_for_param on top
  of the generic methods.
* Add a "data-changed" signal.

liblogdiag/ld-diagram-symbol.c

@@ -8,6 +8,8 @@
  *
  */
 
+#include <string.h>
+
 #include "liblogdiag.h"
 #include "config.h"
 
@@ -65,12 +67,16 @@ ld_diagram_symbol_get_property (GObject *object, guint property_id,
 	GValue *value, GParamSpec *pspec)
 {
 	LdDiagramObject *self;
+	GValue tmp_value;
 
 	self = LD_DIAGRAM_OBJECT (object);
 	switch (property_id)
 	{
 	case PROP_CLASS:
-		ld_diagram_object_get_data_for_param (self, value, pspec);
+		memset (&tmp_value, 0, sizeof (GValue));
+		ld_diagram_object_get_data_for_param (self, &tmp_value, pspec);
+		g_value_copy (&tmp_value, value);
+		g_value_unset (&tmp_value);
 		break;
 	default:
 		G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);