adjustments to filename increment

- start with no number (Untitled0 -> Untitled.ipynb)
- copy of copy increments instead of adding another `-Copy` (copy Foo-Copy1.ipynb gives Foo-Copy2.ipynb, not Foo-Copy1-Copy1.ipynb)
- copy file to new folder starts with the original filename, instead of unconditional `-Copy0`
Merge pull request #6915 from minrk/contents-no-0

use Untitled instead of Untitled0