Add class to provide Undo/Redo support.

This will replace the one in korganizer because it's written with async operations in mind.

It also has less code, it's simpler, and has good error reporting, instead of just true|false.

Made room for two improvments:
- If you create an incidence and don't send invitation, then undo shouldn't ask.
- Group operations should be undone atomically, if the second change fails, the first one must be redone.