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/VolumeWii.cpp

@@ -278,6 +278,11 @@ std::string VolumeWii::GetGameID(const Partition& partition) const
   return DecodeString(id);
 }
 
+std::string VolumeWii::GetGameTDBID(const Partition& partition) const
+{
+  return GetGameID(partition);
+}
+
 Region VolumeWii::GetRegion() const
 {
   const std::optional<u32> region_code = m_reader->ReadSwapped<u32>(0x4E000);