allow to specify a fallback icon for icon selection in the gui * files-dock-widget.cc (files_dock_widget): provide a fallback icon for the pulldown menu for themes that do not contain the new and non-standard icon folder-settings * libgui/src/icons/octave/128x128/folder-settings.png/.svg: new icon by Luis Eduardo Ribeiro Guerra * libgui/src/module.mk, libgui/src/resource.qrc: add new icon * resource-manager.cc/.h (icon): provide fallback icon as third argument