Properly fix linking QtMain on windows

CMake policy CMP0020 (that since CMake 2.8.11 automatically links
QtMain target when set to NEW for WIN32 targets) was discarded because
set in the YarpUseQt5 file.

The "cmake_policy" call is now in the main CMakeLists, so that it is
automatically used in all the project.
Fix CMake warnings caused by qt5_use_modules macro

The macro is now hidden on CMake 2.8.11 or later, by the one defined in
YarpUseQt5.cmake, that just calls target_link_libraries instead.