ControlGroup/Slider: Return state data by value

Makes it less error-prone to get state data from sliders (no need
to pass any locals), and also allows direct assignment, letting the
retrieved data be const.

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

@@ -13,9 +13,14 @@ namespace ControllerEmu
 class Slider : public ControlGroup
 {
 public:
+  struct StateData
+  {
+    ControlState value{};
+  };
+
   Slider(const std::string& name_, const std::string& ui_name_);
   explicit Slider(const std::string& name_);
 
-  void GetState(ControlState* slider);
+  StateData GetState();
 };
 }  // namespace ControllerEmu