refactor: Block unsafe fs::path std::string conversion calls

There is no change in behavior. This just helps prepare for the
transition from boost::filesystem to std::filesystem by avoiding calls
to methods which will be unsafe after the transaction to std::filesystem
to due lack of a boost::filesystem::path::imbue equivalent and inability
to set a predictable locale.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
Co-authored-by: Kiminuo <kiminuo@protonmail.com>
Co-authored-by: MarcoFalke <falke.marco@gmail.com>

src/qt/guiutil.cpp

@@ -647,12 +647,12 @@ void setClipboard(const QString& str)
 
 fs::path qstringToBoostPath(const QString &path)
 {
-    return fs::path(path.toStdString());
+    return fs::u8path(path.toStdString());
 }
 
 QString boostPathToQString(const fs::path &path)
 {
-    return QString::fromStdString(path.string());
+    return QString::fromStdString(path.u8string());
 }
 
 QString NetworkToQString(Network net)