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)

src/qt/intro.cpp

@@ -263,7 +263,7 @@ bool Intro::showIfNeeded(bool& did_show_intro, int64_t& prune_MiB)
      * (to be consistent with bitcoind behavior)
      */
     if(dataDir != GUIUtil::getDefaultDataDirectory()) {
-        gArgs.SoftSetArg("-datadir", GUIUtil::qstringToBoostPath(dataDir).string()); // use OS locale for path setting
+        gArgs.SoftSetArg("-datadir", fs::PathToString(GUIUtil::qstringToBoostPath(dataDir))); // use OS locale for path setting
     }
     return true;
 }