Menu doesn't select the correct item

[savetheclocktower]

When I run the usage example from "$DIALOG" help menu...

"$DIALOG" menu --items '({title = foo;}, {separator = 1;}, {header=1; title = bar;}, {title = baz;})'

…it doesn't return the correct item. For instance, when I select the "baz" item, the returned plist points to the separator.

I believe this commit fixes the issue, but also appears to add a feature. I was hoping to be able to submit a pull request with only the bugfix, but I don't have enough experience with Cocoa to do so confidently.

Submitting this as a separate issue because I think it's of a higher priority than the other stuff that @Bibiko put into #6.