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/CMakeLists.txt

@@ -4,12 +4,13 @@ set(SRCS	Blob.cpp
 			CompressedBlob.cpp
 			DiscScrubber.cpp
 			DriveBlob.cpp
+			Enums.cpp
 			FileBlob.cpp
 			FileMonitor.cpp
 			FileSystemGCWii.cpp
 			Filesystem.cpp
 			NANDContentLoader.cpp
-			VolumeCommon.cpp
+			Volume.cpp
 			VolumeCreator.cpp
 			VolumeDirectory.cpp
 			VolumeGC.cpp