Add undo/redo

[Kangaroux]

Performing a reversible action adds it to the undo stack. If the user presses undo, the change is reversed, and it's moved to the redo stack. Likewise the user can press redo to re-apply it and move it back to the undo stack.

The redo stack should be cleared when the user performs a new action.

• Keep track of actions taken using a stack
• Add a method to Action which undoes the change
• Add a keybind for triggering undo
• Handle undo action
• Add a keybind for triggering redo
• Handle redo action
• Take an inventory of all the things which should be undoable and create actions for things which are missing