WiiSave: Clean up constructors

Move the import/export operation into separate functions, as it doesn't
really make sense for the constructor to do *everything*, including
printing success/failure message boxes.

The existing constructor was split into two: one that takes a path,
and another taking a title ID. This makes it more obvious what is
actually done when a path/TID is passed and also clarifies what
parameters should be passed. (No more magic 0 or "" value.)

Source/Core/DolphinQt2/GameList/GameList.cpp

@@ -260,7 +260,7 @@ void GameList::ExportWiiSave()
 {
   QMessageBox result_dialog(this);
 
-  const bool success = WiiSave::ExportWiiSave(GetSelectedGame()->GetTitleID());
+  const bool success = WiiSave::Export(GetSelectedGame()->GetTitleID());
 
   result_dialog.setIcon(success ? QMessageBox::Information : QMessageBox::Critical);
   result_dialog.setText(success ? tr("Successfully exported save files") :