(preesm) The backend is now considered as VTL specific backend

It prints all actors from the input project, and doesn't use printActors()
anymore
(backends) Remove useless printActors() from AbstractBackend
(util) New method in Result class, for empty result checking
(backends) Starts to implements generation process for non VTL backends
(backends) refactoring on new methods name
(util) Fix a big error in Result class

We can't store static references to unique objects EMPTY, OK, etc because
these objects are always modified (merged with other) when they are used.

Static constants have been replaced by static methods, a new objects is
always created
(backends) Remove another useless method

Also move transformActors() up in the class