Add whitespace-cleanup-mode-file-is-clean-p to update whitespace-cleanup-mode-only-if-initially-clean variable.

This fix the following problem:
1. Open a file with trailing whitespaces
2. Clean-it
3. Save the file
4. Do some more work and introduce some trailing whitespaces
5. Save the file -> whitespace-cleanup was not called

TODO it does not work if after 5. undo is called repeatedly to return
to 1. as it should not do whitespace-cleanup.