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/DynamicProperty.cpp

@@ -215,7 +215,7 @@ bool DynamicProperty::removeDynamicProperty(const char* name)
             throw Base::RuntimeError("property is not dynamic");
         Property *prop = it->property;
         GetApplication().signalRemoveDynamicProperty(*prop);
-        delete prop;
+        Property::destroy(prop);
         index.erase(it);
         return true;
     }