merge doublemelon (#2067)

non-exhaustive (but exhausting) list of changes:

* base laid for multiple window support, but will likely require more work to work correctly
* encapsulation of frontend state for proper multi-instance support
* (JIT still needs a fix for the NDS::Current workaround but we can get there later)
* new, more flexible configuration system

src/frontend/qt_sdl/MPSettingsDialog.h

@@ -25,6 +25,8 @@
 namespace Ui { class MPSettingsDialog; }
 class MPSettingsDialog;
 
+class EmuInstance;
+
 class MPSettingsDialog : public QDialog
 {
     Q_OBJECT
@@ -58,6 +60,7 @@ private slots:
 
 private:
     Ui::MPSettingsDialog* ui;
+    EmuInstance* emuInstance;
 
     QButtonGroup* grpAudioMode;
 };