I've made some changes to the TkKit including:

- TKAPP::register_callback* renamed to
  TKAPP::cb*. The old ones still exist but
  would be wonderful candidates for 'obsolete'.

- Callbacks can now be explicitely created, the
  advantage is a method 'call_str(args: ARRAY{STR}): STR'
  which returns a TCL command to call this callback.

- The new 'bind' command makes use of this:
  bind(evntspec: STR, cb: $TKKIT_CB, args: ARRAY{STR}) now
  simplifies event binding.

- TCL_TK::eval returns STR, no more TUP{INT, STR}. On error, an
  exception of type $TKKIT_EX is raised. Nethertheless, callbacks
  are still required to return tuples.

You see, most of this is cosmetic.

All old sources should compile, though.