Rec-stop only writes a pending (recovery) file (#9215)

This is consistent, since after handling the
`SaveSessionRequested` signal the session remains dirty
(which may be a bug). However since the signal is handled
by the GUI, rec-stop only saves the session if there is a GUI.

It is however somewhat dangerous. Record, quit + no-save (or
changing snapshots w/o saving first) will loose any references
to the recorded data (even though it is still on disk).

Note that "remove last capture" still saves the session
(to prevent references to non-existent sources).

gtk2_ardour/ardour_ui_session.cc

@@ -1324,7 +1324,7 @@ void
 ARDOUR_UI::save_session_at_its_request (std::string snapshot_name)
 {
 	if (_session) {
-		_session->save_state (snapshot_name);
+		_session->save_state (snapshot_name, true);
 	}
 }