Undo could be implemented later in the development a good undo and redo system is not easy and requires some work.
I've always thought that too, but when I actually decided to implement one, it was alot easier than I had assumed.
I rolled my own, but you mentioned you're using Qt, and Qt has its own Undo/Redo framework you can leverage.
You're editor looks nice, btw!
wow I didn't know that but I see it's Qt 5.0, the project its based on Qt 4.8.1, I'll give it a look.
Now I'm focusing on object hierarchy(need to connect signals and slots), final save format and bundled level player(but you can launch your game directly if you prefer)
A new build will be out before easter with at least some new features