Save selection to recent-files in the asynchronous Save cases

Save selection to recent-files in the asynchronous Save cases

Various paths in SAVE or CREATE_FOLDER return from ::should_respond() but
leave an asynchronous process running.  This process checks some things
in the user's selection, for example, 'does the file exist, and if so
do we need to bring up an overwrite-confirmation dialog?'.  When these
async processes complete *and* it is indeed time for the dialog to
be terminated (via the response-requested signal), we also need to
save the selection to the recently-used list - as ::should_respond() does
by itself in the cases when it can request a response immediately.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>