DolphinQt: Reset TAS input slider to default on right-click

This is a feature which existed in DolphinWX. Seems like it got
implemented in DolphinQt for IRWidget/StickWidget but not sliders.

Source/Core/DolphinQt/TAS/TASSlider.h

@@ -0,0 +1,22 @@
+// Copyright 2020 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <QSlider>
+
+class QMouseEvent;
+
+class TASSlider : public QSlider
+{
+public:
+  explicit TASSlider(int default_, QWidget* parent = nullptr);
+  explicit TASSlider(int default_, Qt::Orientation orientation, QWidget* parent = nullptr);
+
+protected:
+  void mouseReleaseEvent(QMouseEvent* event) override;
+
+private:
+  int m_default;
+};