Qt: Clear registers widget on shutdown.

src/citra_qt/debugger/registers.h

@@ -8,6 +8,7 @@
 #include <QTreeWidgetItem>
 
 class QTreeWidget;
+class EmuThread;
 
 class RegistersWidget : public QDockWidget
 {
@@ -20,6 +21,9 @@ public slots:
     void OnDebugModeEntered();
     void OnDebugModeLeft();
 
+    void OnEmulationStarted(EmuThread* emu_thread);
+    void OnEmulationStopped();
+
 private:
     Ui::ARMRegisters cpu_regs_ui;