Keep track of GameTDB ID separately from game ID

The difference between Dolphin's game IDs and GameTDB's game IDs
is that GameTDB uses four characters for non-disc titles, whereas
Dolphin uses six characters for all titles.

This fixes:

- TitleDatabase considering Datel discs to be NHL Hitz 2002
- Gecko code downloading not working for discs with IDs starting with P
- Cover downloading mixing up discs with channels (e.g. Mario Kart Wii
  and Mario Kart Channel) and making extra HTTP requests. (Android was
  actually doing a better job at this than DolphinQt!)

Source/Core/DolphinQt/Config/GeckoCodeWidget.h

@@ -48,6 +48,7 @@ private:
 
   const UICommon::GameFile& m_game;
   std::string m_game_id;
+  std::string m_gametdb_id;
   u16 m_game_revision;
 
   CheatWarningWidget* m_warning;