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.cpp

@@ -168,6 +168,7 @@ void SConfig::SaveInterfaceSettings(IniFile& ini)
   interface->Set("ShowLogConfigWindow", m_InterfaceLogConfigWindow);
   interface->Set("ExtendedFPSInfo", m_InterfaceExtendedFPSInfo);
   interface->Set("ShowActiveTitle", m_show_active_title);
+  interface->Set("ShowDevelopmentWarning", m_show_development_warning);
   interface->Set("ThemeName", theme_name);
   interface->Set("PauseOnFocusLost", m_PauseOnFocusLost);
   interface->Set("DisableTooltips", m_DisableTooltips);
@@ -480,6 +481,7 @@ void SConfig::LoadInterfaceSettings(IniFile& ini)
   interface->Get("ShowLogConfigWindow", &m_InterfaceLogConfigWindow, false);
   interface->Get("ExtendedFPSInfo", &m_InterfaceExtendedFPSInfo, false);
   interface->Get("ShowActiveTitle", &m_show_active_title, true);
+  interface->Get("ShowDevelopmentWarning", &m_show_development_warning, true);
   interface->Get("ThemeName", &theme_name, DEFAULT_THEME_DIR);
   interface->Get("PauseOnFocusLost", &m_PauseOnFocusLost, false);
   interface->Get("DisableTooltips", &m_DisableTooltips, false);

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;