add print_text_after_time to specify time stamp for lines

The proposed API change and amendment to Perl script API would allow
printing lines in the middle of the TextBuffer with an attached
time-stamp. This can be used in the hideshow or dim_nicks script to
dynamically rewrite lines in the visible TextBuffer history. Old
scripts are unaffected and unlikely to have used the API as it used to
break scrollback (see #10).

Note, there might be better ways to alter lines than removing and
reinserting them, but that might be a larger reorganisation of the
fe-text.