Use LXQt translation infrastructure

No LXQt dependency is added. It remains a Qt only app.

To update the translations set the CMake command line option
UPDATE_TRANSLATIONS to true. Ex: cmake -DUPDATE_TRASNLATIONS=ON ..
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=OFF ..

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.

The .desktop files translatable bits are now split by language in the
translations directory. screengrab.desktop.in holds the non-common, not
translatable parts. The LXQtTranslateDesktop.cmake module glues it all, so
only one file (screengrab.desktop) is installed.

The Qt5TranslationLoader.cmake module takes care of automatically produce
code to load the translations.
Updates main applications translations sources