Text-based backends could benefit from String Template: better
maintainability, easier indenting, no mix between code and templates,
etc. This initial commit just puts in place the approach ; everything
has to be done from there.

Ada: When calling an access procedure with no parameter, it seems
mandatory to add ".all" to the call. Not understood why it is not needed
when the procedure has parameters.

- The auto-generated GUIs currently do not support parameterless
interfaces, so they must not appear in the "mini-cv.aadl" file. They can
be handled directly by the gui code.
- Support of Sync RIs, there are two cases (1) the calls to set/reset
  timers, that can be handled directly by the gui code and (2) the calls
to real external code, that has to be provided by the user and linked
with the shared library. An example of this second case is included in
demo-simu.

Another test case to stress the new "simulation" interface.

When generating code for simulation/shared lib, the functions returning
the internal variables must return a pointer to the variables, not the
variables themselves (which happened with records even if the variables
were declared as aliased)