Turn Backend into a sensible classoid.

Nearly every part of the code that ever handles a full backend
structure has historically done it using a pair of pointer variables,
one pointing at a constant struct full of function pointers, and the
other pointing to a 'void *' state object that's passed to each of
those.

While I'm modernising the rest of the code, this seems like a good
time to turn that into the same more or less type-safe and less
cumbersome system as I'm using for other parts of the code, such as
Socket, Plug, BinaryPacketProtocol and so forth: the Backend structure
contains a vtable pointer, and a system of macro wrappers handles
dispatching through that vtable.

terminal.h

@@ -227,8 +227,7 @@ struct terminal_tag {
     wchar_t *paste_buffer;
     int paste_len, paste_pos;
 
-    void (*resize_fn)(void *, int, int);
-    void *resize_ctx;
+    Backend *backend;
 
     Ldisc *ldisc;