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/VolumeGC.cpp

@@ -272,6 +272,11 @@ u8 CVolumeGC::GetDiscNumber() const
 	return disc_number;
 }
 
+IVolume::EPlatform CVolumeGC::GetVolumeType() const
+{
+	return GAMECUBE_DISC;
+}
+
 bool CVolumeGC::LoadBannerFile() const
 {
 	// The methods GetNames, GetDescriptions, GetCompany and GetBanner