InputCommon / DolphinQt / Core: Add a "RelativeMouse" input which provides the raw delta mouse input

Co-authored-by: Jordan Woyak <jordan.woyak@gmail.com>

Source/Core/InputCommon/ControllerInterface/Xlib/XInput2.h

@@ -30,6 +30,7 @@ private:
     unsigned int buttons;
     Common::Vec2 cursor;
     Common::Vec2 axis;
+    Common::Vec2 relative_mouse;
   };
 
   class Key : public Input
@@ -91,6 +92,21 @@ private:
     std::string name;
   };
 
+  class RelativeMouse : public Input
+  {
+  public:
+    std::string GetName() const override { return name; }
+    bool IsDetectable() const override { return false; }
+    RelativeMouse(u8 index, bool positive, const float* axis);
+    ControlState GetState() const override;
+
+  private:
+    const float* m_axis;
+    const u8 m_index;
+    const bool m_positive;
+    std::string name;
+  };
+
 private:
   void SelectEventsForDevice(XIEventMask* mask, int deviceid);
   void UpdateCursor();