Refactor syntax unicode ep constructors1

Updates on
``
bool MCButton::selectedchunk(MCStringRef& r_string)
bool MCButton::selectedline(MCStringRef& r_string)
bool MCField::returnchunk(int4 p_si, int4 p_ei, MCStringRef& r_chunk)
bool MCField::returnline(int4 si, int4 ei, MCStringRef& r_string)
``