Move DiscIO enums to a new file

At first there weren't many enums in Volume.h, but the number has been
growing, and I'm planning to add one more for regions. To not make
Volume.h too large, and to avoid needing to include Volume.h in code
that doesn't use volume objects, I'm moving the enums to a new file.
I'm also turning them into enum classes while I'm at it.

Source/Core/DiscIO/Blob.h

@@ -22,7 +22,7 @@
 
 namespace DiscIO
 {
-// Increment CACHE_REVISION if the enum below is modified (ISOFile.cpp & GameFile.cpp)
+// Increment CACHE_REVISION (ISOFile.cpp & GameFile.cpp) if the enum below is modified
 enum class BlobType
 {
   PLAIN,