made the get_native_font() function return a shared_ptr_thread_safe object instead
of a raw pointer, and also made the letter object noncopyable (as it should have been).
I also moved some code from the widgets header file to the cpp file.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402574