Implement Undo/Redo functionality

[davidfokkema]

This might be a pain. You have to use Qt's Undo framework where you have to transfer functionality to undo() and redo() methods in a QUndoCommand class. You can push those on the stack. In principle, this can be nice, but in practice I fear that a collection of regular methods in a Tailor class have to be moved to a potentially long list of QUndoCommand classes. Don't know yet how that will affect the code base.