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/UICommon/GameFile.h

@@ -67,6 +67,7 @@ public:
   std::vector<DiscIO::Language> GetLanguages() const;
   const std::string& GetInternalName() const { return m_internal_name; }
   const std::string& GetGameID() const { return m_game_id; }
+  const std::string& GetGameTDBID() const { return m_gametdb_id; }
   u64 GetTitleID() const { return m_title_id; }
   const std::string& GetMakerID() const { return m_maker_id; }
   u16 GetRevision() const { return m_revision; }
@@ -120,6 +121,7 @@ private:
   std::map<DiscIO::Language, std::string> m_descriptions{};
   std::string m_internal_name{};
   std::string m_game_id{};
+  std::string m_gametdb_id{};
   u64 m_title_id{};
   std::string m_maker_id{};