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/Core/ConfigManager.h

@@ -257,6 +257,7 @@ struct SConfig : NonCopyable
   bool m_InterfaceLogConfigWindow;
   bool m_InterfaceExtendedFPSInfo;
   bool m_show_active_title = false;
+  bool m_show_development_warning;
 
   bool m_ListDrives;
   bool m_ListWad;