It is not possible to share the stack between SDL and MSC handlers, they
live in separate threads.

When working with a dll, the API offers the possibility to return to a
previously stored global system state, enabling (infinite) undo
capability - yet to be implemented.

Depending on the state of the SDL model, some buttons for sending
messages are now disabled, if the messages are not explicitely handled
by the model.

All data is now displayed in a tree widget

Reuse the ASN.1 Editor tree view to display the shared library internal
variables.

The mapping of the ctypes value (returned by the shared lib) to a swig
pointer was incorrect. The return value of the Ada function had to be
set in ctypes to POINTER(c_char) and not to c_char_p (which returned a
python 'str' objet with messy values inside).

The SDL handler is parsing the SDL file, so it can read the list of
declared variables. Using the shared lib it can then read the actual
value of each parameter at runtime. An event (signal) is received by a
slot when an message (call of a PI) has ben processed. This is the
placeholder to put the read of the DLL.