Adds and uses QtTranslate cmake module for handling .ts translations

We now have the ability to choose when to update (extract) the
translations from the sources files.

To update the translations set the CMake command line option
UPDATE_TRANSLATIONS to true. Ex: cmake -DUPDATE_TRASNLATIONS=Yes ..
The translations will be extracted and also compiled.
To return to the compilation only mode just set the UPDATE_TRANSLATIONS
option to false. Ex: cmake -DCMAKE_UPDATE_TRANSLATIONS=No ..

Warning: When UPDATE_TRANSLATIONS is enabled a clean command will also
clean the generated .ts files. The CLEAN_NO_CUSTOM property should solve
that problem. But it didn't, at least for now. So, after updating the
translations, disable UPDATE_TRANSLATIONS right away.