Always use visible text in GameList

Depending upon the desktop colour scheme, the light/dark
GameList backgrounds can cause the always white text
to become unreadble.

Use the common luminance approximation algorithm to
determine whether black text should be used instead.

Source/Core/DolphinWX/GameListCtrl.h

@@ -73,7 +73,7 @@ private:
   void InitBitmaps();
   void UpdateItemAtColumn(long _Index, int column);
   void InsertItemInReportView(long _Index);
-  void SetBackgroundColor();
+  void SetColors();
   void ScanForISOs();
 
   // events