HLE/FS: Implemented FSFile::OpenSubFile.

The File class now holds a list of connected sessions along with data unique to each session.

A subfile is a window into an existing file. They have a few limitations compared to normal files:

* They can't be written to.
* They can't be flushed.
* Their size can not be changed.
* New subfiles can't be created from another subfile.

src/core/hle/kernel/hle_ipc.h

@@ -54,7 +54,7 @@ public:
      * associated ServerSession.
      * @param server_session ServerSession associated with the connection.
      */
-    void ClientDisconnected(SharedPtr<ServerSession> server_session);
+    virtual void ClientDisconnected(SharedPtr<ServerSession> server_session);
 
 protected:
     /// List of sessions that are connected to this handler.