Eina unicode: add eina_unicode_unicode_to_utf8_range

Eina unicode: add eina_unicode_unicode_to_utf8_range

Required some special treatment to get words (substring) out of an
input Unicode string to a utf8 one. This saves the trouble converting
the whole string: you input an offset in the Eina_Unicode array, and
provide the required length. That's is, now you can extract words and
whatnot in utf8 form.

To save code I wrapped the original one to get the whole length.