`py--skip-to-comment-or-semicolon', return point if successful

related minor fixes
window-management WRT execute-buffer/file commands fixed
Some commands not so useful made common functions, like
  py-statement-opens-block-p
  py-statement-opens-clause-p ...

Internally used functions preferably are prefixed `py--':
  py--statement-opens-block-p
  py--statement-opens-clause-p ...
use `py--imenu-create-index', fix