More special cases for deleted files
slide-delete is now "sd", not "sx"

I always mistakenly type "sd" instead of "sx". I guess "sx" just wasn't
a good mnemonic.
allow save and transform to duplicate slides

'insert' could easily duplicate slides, but the alternate workflow of
modifying the next slide to be identical to the previous one would not
work because git would complain that there is nothing to commit.