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/Core/ConfigManager.h

@@ -367,12 +367,13 @@ private:
   void LoadAutoUpdateSettings(IniFile& ini);
   void LoadJitDebugSettings(IniFile& ini);
 
-  void SetRunningGameMetadata(const std::string& game_id, u64 title_id, u16 revision,
-                              Core::TitleDatabase::TitleType type);
+  void SetRunningGameMetadata(const std::string& game_id, const std::string& gametdb_id,
+                              u64 title_id, u16 revision);
 
   static SConfig* m_Instance;
 
   std::string m_game_id;
+  std::string m_gametdb_id;
   std::string m_title_description;
   u64 m_title_id;
   u16 m_revision;