When converting plain_text to '*' in retrieve_surrounding_cb,
always convert it to '*' in 1 byte unit.
2 byte character is converted to "* *"
and 3 byte character is converted to "* * *"
However, this does not match the number of '*' printed in the entry.
Because, '*' in the entry is printed according to number of unicode characters.
This patch converts plain_text into unicode units
when converting plain_text to '*'