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/VolumeWiiCrypted.cpp

@@ -238,9 +238,9 @@ std::string CVolumeWiiCrypted::GetApploaderDate() const
 	return date;
 }
 
-bool CVolumeWiiCrypted::IsWiiDisc() const
+IVolume::EPlatform CVolumeWiiCrypted::GetVolumeType() const
 {
-	return true;
+	return WII_DISC;
 }
 
 u8 CVolumeWiiCrypted::GetDiscNumber() const