Merging desktop icon settings

The icons of "applications-math.desktop" and "setxkbmap-gadget.desktop" will be
installed on Desktop when LXDE starts up.

I've used an utility "xdg-desktop-icon". So we don't have to worry whether the entries
are already existing or not, or whether the folder is "Desktop" or "デスクトップ", etc.
I hope this is a better way than preparing /etc/skel/Desktop, because users sometimes
delete icons by accident and need to copy again when using persistent media.