Automatic save, shadow backup/restore (#120)

Hi !

You have done a very impressive tool ! Thanks a lot to had let us to use it freely for non commercial usages.

I have an important request. Sometimes, the application crashes on some actions. I had not report it because I can't specify why it's happens exactly. On some complex diagrams, when you click on Intelligent reorganization, the application crashes.

It will be good to have a functionality of auto-saving (with specified time) and a shadow backup which be retrieved at the start of the application if the last project has not been correctly saved. I had lost hours of work by simply forgetting to save my work before to push this button.

Make it available as quick as possible should be good.

Except that, your program is beautiful. Bravo ! ;-)