Fix broken swap-file on windows

Also fix alternative-directory swap-files on all platforms.

The swap-file problem was QDir::separator() usage in stead of '/'.
QUrl always uses '/' regardless of platform.

The alternative-directory swap-files was trying to use the
raw hash of the document unconverted to hex.

There is still one QDir::separator() usage in:
src/document/katedocument.cpp that should be checked!

CCMAIL: kwrite-devel@kde.org