Save deletes undo history in ParseGenApp #52

BUG

When clicking File->Saveor Generate in ParseGenApp the undo history is lost.

This makes it hard to quickly test a new grammar entry and then rollback to pre-edit condition.

The app should maintain undo history while it is running in the same document.

Some research notes on issue

• NSTextView is bound to grammar as KVO in interface builder
• NSTextView editable is bound to busy as KVO in interface builder
• Saving doc caret jumps to top of doc
• When saving NSTextView looses focus
• Disable busy logic keeps undo history after generate but still broken for File->Save

- (void)done {
    ...
//    self.busy = NO;
//    [self focusTextView];
    ...
}

- (IBAction)generate:(id)sender {
    ...
   //self.busy = YES;
    ...
}