Qt: Implement relative mouse mode

src/duckstation-qt/qthostinterface.h

@@ -139,6 +139,7 @@ Q_SIGNALS:
   void runningGameChanged(const QString& filename, const QString& game_code, const QString& game_title);
   void exitRequested();
   void inputProfileLoaded();
+  void mouseModeRequested(bool relative, bool hide_cursor);
 
 public Q_SLOTS:
   void setDefaultSettings();
@@ -203,6 +204,8 @@ protected:
   void SetDefaultSettings(SettingsInterface& si) override;
   void UpdateInputMap() override;
 
+  void SetMouseMode(bool relative, bool hide_cursor) override;
+
 private:
   enum : u32
   {