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/MainWindow.cpp

@@ -295,7 +295,7 @@ void MainWindow::Play()
     }
     else
     {
-      QString default_path = Settings::Instance().GetDefaultGame();
+      auto default_path = QString::fromStdString(SConfig::GetInstance().m_strDefaultISO);
       if (!default_path.isEmpty() && QFile::exists(default_path))
       {
         StartGame(default_path);
@@ -319,7 +319,7 @@ bool MainWindow::Stop()
   if (!Core::IsRunning())
     return true;
 
-  if (Settings::Instance().GetConfirmStop())
+  if (SConfig::GetInstance().bConfirmStop)
   {
     const Core::State state = Core::GetState();
     // Set to false when Netplay is running as a CPU thread
@@ -430,8 +430,7 @@ void MainWindow::StartGame(const QString& path)
 
 void MainWindow::ShowRenderWidget()
 {
-  auto& settings = Settings::Instance();
-  if (settings.GetRenderToMain())
+  if (SConfig::GetInstance().bRenderToMain)
   {
     // If we're rendering to main, add it to the stack and update our title when necessary.
     m_rendering_to_main = true;
@@ -442,13 +441,13 @@ void MainWindow::ShowRenderWidget()
   {
     // Otherwise, just show it.
     m_rendering_to_main = false;
-    if (settings.GetFullScreen())
+    if (SConfig::GetInstance().bFullscreen)
     {
       m_render_widget->showFullScreen();
     }
     else
     {
-      m_render_widget->resize(settings.GetRenderWindowSize());
+      m_render_widget->resize(640, 480);
       m_render_widget->showNormal();
     }
   }