make shellQuote() nicer

- unify unix and windows paths
- on windows:
  - we don't need to care for trailing backslashes, as we never append
    directly to quoted strings
  - simplify, as this doesn't need to be fast.
    unfortunately, it's impossible to use
      ret.replace(QRegExp(QLatin1String("(\\\\*)$")), QLatin1String("\"\\1"));
    as that matches foo\ twice (once one backslash and once an empty
    string).