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.
Cleanup
Replace deprecated OUTPUT_PATH variables with OUTPUT_DIRECTORY equivalents

This time do not use per-config variables, but let CMake handle them.

DLLs are considered "RUNTIME" on windows, and therefore, from this patch,
they will end in the "bin" directory. Therefore install commands are update
to reflect this change.
Fix Qt5 executables and plugins

The path is now different for build-tree and install-tree for multi
config generators, but the executables now import both paths.