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.cpp

@@ -26,8 +26,8 @@
 #include "UICommon/GameFile.h"
 
 GeckoCodeWidget::GeckoCodeWidget(const UICommon::GameFile& game, bool restart_required)
-    : m_game(game), m_game_id(game.GetGameID()), m_game_revision(game.GetRevision()),
-      m_restart_required(restart_required)
+    : m_game(game), m_game_id(game.GetGameID()), m_gametdb_id(game.GetGameTDBID()),
+      m_game_revision(game.GetRevision()), m_restart_required(restart_required)
 {
   CreateWidgets();
   ConnectWidgets();
@@ -251,7 +251,7 @@ void GeckoCodeWidget::DownloadCodes()
 {
   bool success;
 
-  std::vector<Gecko::GeckoCode> codes = Gecko::DownloadCodes(m_game_id, &success);
+  std::vector<Gecko::GeckoCode> codes = Gecko::DownloadCodes(m_gametdb_id, &success);
 
   if (!success)
   {