GameListCtrl: Reimplement Custom Paint to remove column lines.

This leaves system themes enabled and instead just repaints the
list view background to eliminate the column ruler lines.

Source/Core/DolphinWX/GameListCtrl.h

@@ -61,6 +61,10 @@ public:
     NUMBER_OF_COLUMN
   };
 
+#ifdef __WXMSW__
+  bool MSWOnNotify(int id, WXLPARAM lparam, WXLPARAM* result) override;
+#endif
+
 private:
   std::vector<int> m_FlagImageIndex;
   std::vector<int> m_PlatformImageIndex;