This defines a replace_placeholders() function, taking as arguments
a text string and either a timestamp or a sequence / point pair. It
uses the latter arguments to find metadata from which it can extract
relevant information and replace it into the text string wherever the
appropriate placeholders appear. For instance, given a call such as
replace_placeholders('The position is @POS@', NULL, 11, 2600) it will
replace '@POS@' with the position of point 2600 in sequence 11, if it
exists (or leave the placeholder untouched otherwise).
A scan_placeholders() procedure is also defined, which calls the above
function on the entire event log.
Fixes#229.
