Don’t unnecessarily escape backslashes in Win32 paths

Under ::CommandLineToArgvW() rules, the backslash character only gets special treatment if it's part of a sequence of backslashes terminated by a double-quote. So when checking to see if a string needs Win32 escaping, it’s sufficient to just simply check for the presence of a double quote character.

This allows paths like "foo\bar" to be recognised as “sensible” paths, which don’t require the full escaping.

Fixes #700