DolphinQt2: replace Settings with SConfig where possible

Changes:
- `ShowDevelopmentWarning` is now under the '[Interface]' group in
  Dolphin.ini, with other interface-related settings. So, whoever uses
  DolphinQt will have to edit that manually again. Sorry!
- Game search paths and the last file are now shared properly with
  DolphinWX
- Qt-only preferences like "Preferred View: list/table" are now
  stored using the platform's native settings storage, rather than in
  UI.ini

Source/Core/DolphinQt2/Config/SettingsWindow.cpp

@@ -72,7 +72,6 @@ void SettingsWindow::SetupSettingsWidget()
 
 void SettingsWindow::AddCategoryToList(const QString& title, const std::string& icon_name)
 {
-  QString dir = Settings::Instance().GetThemeDir();
   QListWidgetItem* button = new QListWidgetItem();
   button->setText(title);
   button->setTextAlignment(Qt::AlignVCenter);