Update application menu name in layout configs

xfce4-panel switched to rDNS names for icons in Xfce 4.16 (xfce4-panel commit c6f68e97) and no longer provides files with the old name.

This gives error icon when switching to the provided layouts if the icon theme chosen doesn't have a file with the old name.

This MR updates the icon names for the applications menu to use the newer name, since this icon should always be provided by xfce4-panel itself.