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/DiscIO/VolumeWad.cpp

@@ -105,6 +105,11 @@ std::string VolumeWAD::GetGameID(const Partition& partition) const
   return m_tmd.GetGameID();
 }
 
+std::string VolumeWAD::GetGameTDBID(const Partition& partition) const
+{
+  return m_tmd.GetGameTDBID();
+}
+
 std::string VolumeWAD::GetMakerID(const Partition& partition) const
 {
   char temp[2];