ControllerEmu: Add new "input override" system

Source/Core/InputCommon/ControllerEmu/ControlGroup/Slider.h

@@ -23,6 +23,9 @@ public:
   explicit Slider(const std::string& name_);
 
   StateData GetState() const;
+  StateData GetState(const InputOverrideFunction& override_func) const;
+
+  static constexpr const char* X_INPUT_OVERRIDE = "X";
 
 private:
   SettingValue<double> m_deadzone_setting;