qt: Do proper boost::path conversion

Convert from QString unicode from/to the OS-dependent locale as used by boost::filesystem::path as needed.

Should solve #3916.

Test builds can be found here:
https://download.visucore.com/bitcoin/bitcoin-2014_03_qt_unicode_path.zip
Signature:
https://download.visucore.com/bitcoin/bitcoin-2014_03_qt_unicode_path.zip.sig