Filesystem: Replace FileInfo struct with interface

GC/Wii filesystem internals shouldn't be exposed to other classes.
This change isn't especially useful by itself, but it opens up the
way for some neat stuff in the following commits.

Source/Core/DiscIO/Filesystem.cpp

@@ -9,6 +9,10 @@
 
 namespace DiscIO
 {
+FileInfo::~FileInfo()
+{
+}
+
 FileSystem::FileSystem(const Volume* _rVolume, const Partition& partition)
     : m_rVolume(_rVolume), m_partition(partition)
 {