App: make deleting dynamic property safer

Dynamic property can be removed at any time, even during triggering of
onChanged() signal of the removing property. This patch introduced
static function Property::destroy() to make it safer by queueing any
removed property, and only deleting them when no onChanged() call is
active.

src/App/Property.h

@@ -104,6 +104,9 @@ public:
     Property();
     virtual ~Property();
 
+    /// For safe deleting of a dynamic property
+    static void destroy(Property *p);
+
     /** This method is used to get the size of objects
      * It is not meant to have the exact size, it is more or less an estimation
      * which runs fast! Is it two bytes or a GB?