Move a function back to string_utils.hpp
Restore some commented-out code
Rename utils::trancate_as_ucs4string to utf8::truncate_as_ucs4
Further reorganise unicode.hpp

Create a unicode_cast<utf8::string, ucs4::char_t> specialisation.
Move remaining functions in the utils namespace to implemenation.
Merge branch 'master' of github.com:wesnoth/wesnoth