qt: Do proper boost::path conversion

Convert from QString unicode from/to the OS-dependent locale as used by boost::filesystem::path as needed.

This should make data directories with special characters in Windows work.

Should solve #3916 and #3905.

Test builds (windows) can be found here:
https://download.visucore.com/bitcoin/bitcoin-2014_03_qt_unicode_path_v3.zip
Signature:
https://download.visucore.com/bitcoin/bitcoin-2014_03_qt_unicode_path_v3.zip.sig