Retrieves a sequence of characters.
The characters that are retrieved are those characters at positions
start_pos up to, but not including
end_pos is negative,
then the characters retrieved are those characters from
the end of the text.
Note that positions are specified in characters, not bytes.
A pointer to the contents of the widget as a
string. This string is allocated by the
and should be freed by the caller.
|The caller of the method takes ownership of the returned data, and is responsible for freeing it.
|The value is a NUL terminated UTF-8 string.