QUndoStack: Add API to give it a concept of "dirty" to be able to create editor applications conveniently.

Most editor applications implement the concept of "Dirty" by tracking the stack index/position of QUndoStack: This triggers insidious bugs:

1) push command - save - undo - push another command: Appears as clean as the stack index equals that at the time of saving
2) Doing a sequence of commands that are merged:
push some commands - save - push further commands of the same kind: Appears as clean as all commands are merged (see parent task)

Thus it would be nice if the QUndoStack did the book-keeping of "dirty" in a consistent way.