Volume: Return volume type as an enum

ISOFile and GameFile were using IsWiiDisc() and IsWadFile() to set
an enum value. The volume might as well return an enum directly.

I increased the Qt CACHE_REVISION because m_platform now is saved as u32
instead of int, but increasing the wx CACHE_REVISION is not necessary.

Source/Core/DiscIO/VolumeDirectory.h

@@ -47,7 +47,7 @@ public:
 	u32 GetFSTSize() const override;
 
 	std::string GetApploaderDate() const override;
-	bool IsWiiDisc() const override;
+	EPlatform GetVolumeType() const override;
 
 	ECountry GetCountry() const override;