Enhancement: Rework Callback Logic

[StormFX]

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

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