imported notebook name can clash with existing notebooks

[gordonwoodhull]

Possibly when importing notebooks, we should do the same name checks that we do when forking.

For "Import Notebook from File", we could suggest the new name; "Import External Notebooks" already names them automatically, so it could check the folder name, if specified, like folder forking does, or check the individual names.

Would be nice to never clash. Not sure how important this is, though.