Add a setFailed method to QUndoCommand

If a (possibly) complex operation can fail, it is currently rather complicated to use it with the QUndo framework as there is no way to tell the framework to simply forget about the current action.

Changing the signature of "redo" to return a boolean won't be an option, so a protected "setFailed" property could be added to QUndoCommand which could then be evaluated by the QUndoStack.