or_changes Monitoring changes to the tree

A powerful feature is the ability to monitor modifications to a tree. This is done through the Qtilities::Core::Interfaces::IModificationNotifier interface. This interface allows any object in the tree to broadcast any changes to its modification state. An example of where this is useful is where a tree is part of a project. In that case we want to indicate to the user that the project must be saved as soon as the tree structure or any of the objects in the tree ch