fully-translatable interface. Italian translation.

I:
* added hopefully all the needed tr() to surround all the strings.
* enabled translation by adding the needed qt-related code in main.cpp
* enabled auto-building of translation at build time in pencil.pro
* added Italian translation (pencil2d_it.ts) and added it to pencil.pro
(If you'll credit the translator somewhere, Andrea Primiani <primiani@dag.it>, it would be greatly appreciated. Usually (in other programs) there is an AUTHORS file listing all contributors).

Please see bug #241.

If I'll find any other strings needing tr() I'll push the change here unless you'll have already merged the branch.

Thanks!