qt: Do proper boost::path conversion

Convert from QString unicode from/to the OS-dependent locale
as used by boost::filesystem::path as needed.

Solves #3916.

src/qt/guiutil.h

@@ -11,6 +11,8 @@
 #include <QTableView>
 #include <QHeaderView>
 
+#include <boost/filesystem.hpp>
+
 class QValidatedLineEdit;
 class SendCoinsRecipient;
 
@@ -164,6 +166,12 @@ namespace GUIUtil
     /** Restore window size and position */
     void restoreWindowGeometry(const QString& strSetting, const QSize &defaultSizeIn, QWidget *parent);
 
+    /* Convert QString to OS specific boost path through UTF-8 */
+    boost::filesystem::path qstringToBoostPath(const QString &path);
+
+    /* Convert OS specific boost path to QString through UTF-8 */
+    QString boostPathToQString(const boost::filesystem::path &path);
+
 } // namespace GUIUtil
 
 #endif // GUIUTIL_H