DiscIO: Merge WiiWAD into VolumeWAD

These two classes are very similar, so let's merge them.

Source/Core/DiscIO/Volume.h

@@ -74,6 +74,7 @@ public:
   {
     return INVALID_CERT_CHAIN;
   }
+  virtual std::vector<u8> GetContent(u16 index) const { return {}; }
   virtual std::vector<u64> GetContentOffsets() const { return {}; }
   // Returns a non-owning pointer. Returns nullptr if the file system couldn't be read.
   virtual const FileSystem* GetFileSystem(const Partition& partition) const = 0;