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/DateTimeDialog.h

@@ -26,6 +26,8 @@
 namespace Ui {class DateTimeDialog; }
 class DateTimeDialog;
 
+class EmuInstance;
+
 class DateTimeDialog : public QDialog
 {
     Q_OBJECT
@@ -63,6 +65,7 @@ private slots:
 
 private:
     Ui::DateTimeDialog* ui;
+    EmuInstance* emuInstance;
 
     QDateTime customTime;
 };