Introduce 'spawn_command_async()', with error reporting

This should clarify bugs such as ID 3614887,
https://sourceforge.net/tracker/?func=detail&atid=894869&aid=3614887&group_id=180858
menu: Add accelerators to Main menu - ID: 3607870

With this patch we can add a underscore (_) before the letter we want to
use as accelerator on Name of Desktop Entry section.

Please see Feature Request 3592406.
src/panel: update copyright
configurator: use less casting

The GTK_*() functions/macros can have checks in them.
lxpanel_launch_app(): allow starting in a different workdir

Should fix bug ID 3598840,
https://sourceforge.net/tracker/?func=detail&atid=894869&aid=3598840&group_id=180858