Save last-opened file path as local-file format

It's stored in the format of '/home/user/Models/Basic' rather than 'file:///home/user/Models/Basic'. The QML FileDialog class expects the latter format though.

Contributes to issue CURA-3297.

cura/CuraApplication.py

@@ -400,7 +400,7 @@ class CuraApplication(QtApplication):
 
     @pyqtSlot(str, str)
     def setDefaultPath(self, key, default_path):
-        Preferences.getInstance().setValue("local_file/%s" % key, default_path)
+        Preferences.getInstance().setValue("local_file/%s" % key, QUrl.toLocalFile(default_path))
 
     ##  Handle loading of all plugin types (and the backend explicitly)
     #   \sa PluginRegistery