Peritext undo/redo

[streamich]

Undo/redo module

• Implement undo and redo stacks.
• Make the module sufficiently generic, so it can work across multiple documents.
• Ability to store non-persistent events, such as mouse/cursor/view changes.\
• Integrate with document native undo/redo stack
• Add ability to have editor stand-alone undo/redo stack