The current callback logic needs to be rework in a way that makes it more efficient.

• #316
• Add a new RegisterCallback method to the Group API.
• Migrate all callbacks to be registered and stored at the group level.
• Move the FireCB function to a private method.
• Move the firing of callbacks from ReSkin to SetOption.
• Implement a warning system for add-ons to notify authors/users when a deprecated callback registration method is called.