Display an alert box and wait for user's choice.
If a single image is in the selection, it is used as an icon for the alert box.
'title=[G'MIC Alert]' and 'message=This is an alert box.'.
G'MIC is an open-source software distributed under the
CeCILL free software licenses (LGPL-like and/or
Copyrights (C) Since July 2008,
David Tschumperlé - GREYC UMR CNRS 6072, Image Team.