Add warning when opening tutorial

[pgleeson]

When a user selects New in the file menu, they get a warning that the current model will be deleted.
Conversely, when a user selects Help -> Tutorial 1 (maybe expecting a pop up), the current model is deleted without warning. There should be a confirmation dialog...

[pgleeson]

Also, maybe it's just me (and I appreciate the need to give feedback on progress...) but if I see there are 82 steps in a tutorial, I'm probably not going to do it...