IOS/ES: Handle contexts properly

This changes the IOS code to handle ES contexts inside of ES, instead
of leaking out implementation details into the IPC request dispatcher.

The intent is to clarify what's shared between every single ES context,
and what is specific to an ES context. (Not much.) This should reduce
the number of static members in the ES class.

The other changes are there just because we now keep track of the
IPC FD inside of ES.

Future plans:

* After the WAD direct launch hack is dropped, the title context
  will be made a class member.

* Have proper function prototypes, instead of having every single one
  of them take ioctlv requests. This will allow reusing IOS code in
  other parts of the Dolphin codebase without having to construct
  ioctlv requests.

Source/Core/Core/IOS/DeviceStub.h

@@ -22,7 +22,6 @@ public:
   Stub(u32 device_id, const std::string& device_name);
 
   ReturnCode Open(const OpenRequest& request) override;
-  void Close() override;
   IPCCommandResult IOCtl(const IOCtlRequest& request) override;
   IPCCommandResult IOCtlV(const IOCtlVRequest& request) override;
 };