Rudimentary post-processing of exported files.

Rudimentary post-processing of exported files.

Export format contains a string to be passed to system() after expanding
%1, %2, & %3 via string_compose() to the full path & filename, containing
directory, and basename respectively. No error-checking or any niceties
like that - real programmers will of course always type the command
correctly, and know to watch Ardour's standard output for the results...