Add undo/redo system

[vlazed]

This issue shall report a method to restoring the state of an animation ingame.

A proper undo/redo system shall have the following properties:

• A configurable limit. For example, we can store at least 50 states in singleplayer, and around 10-20 states in multiplayer
• The states that changed for the selected entity/group
• UI to inform the state change

State storage

For undos and redos, we shall store the following changes to state:

• Record Keyframe
• Move Keyframe
• Copy Keyframe
• Update Easing/Interpolation
• Entity creation/deletes

I think these are a good basis for newer versions. Entity creation/deletion, however, adds a complexity that should also inform the first four bulletpoints related to keyframe operations