Core: Remove some header inclusions in header files

Replaces them with forward declarations of used types, or removes them entirely if they aren't used at all. This also replaces certain Common headers with less inclusive ones (in terms of definitions they pull in).

Source/Core/Core/HW/DVDInterface.h

@@ -7,9 +7,9 @@
 #include <string>
 
 #include "Common/CommonTypes.h"
-#include "DiscIO/Volume.h"
 
 class PointerWrap;
+namespace DiscIO { class IVolume; }
 namespace MMIO { class Mapping; }
 
 namespace DVDInterface
@@ -79,7 +79,7 @@ enum DICommand
 	DVDLowAudioBufferConfig = 0xe4
 };
 
-enum DIInterruptType
+enum DIInterruptType : int
 {
 	INT_DEINT = 0,
 	INT_TCINT = 1,