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/GeckoCodeConfig.cpp

@@ -16,21 +16,9 @@
 
 namespace Gecko
 {
-std::vector<GeckoCode> DownloadCodes(std::string gameid, bool* succeeded)
+std::vector<GeckoCode> DownloadCodes(std::string gametdb_id, bool* succeeded)
 {
-  switch (gameid[0])
-  {
-  case 'R':
-  case 'S':
-  case 'G':
-    break;
-  default:
-    // All channels (WiiWare, VirtualConsole, etc) are identified by their first four characters
-    gameid = gameid.substr(0, 4);
-    break;
-  }
-
-  std::string endpoint{"https://www.geckocodes.org/txt.php?txt=" + gameid};
+  std::string endpoint{"https://www.geckocodes.org/txt.php?txt=" + gametdb_id};
   Common::HttpRequest http;
 
   // Circumvent high-tech DDOS protection