* src/pcreshell.cc (translate_to_char_offset): New function that translates
the error offset returned by pcre_compile() to a character offset into the
original regex.  This became necessary since due to the recent introduction
of escape_non_ascii(), and is more robust anyway.

* src/mainwindow.cc (on_exec_search): Deal with character offsets instead of
byte offsets.  Also, set the regex entry's selection on the character where
the error occurred.