Ltsp-manager/Update image

After the initial installation, and also every time that you have server updates (new software etc) that you want to push to your clients, you need to open LTSP Manager and go to the Server > LTSP image > Update menu. This menu will open a terminal and it will run the `ltsp-update-image --cleanup /` command, that will clone your server installation into /opt/ltsp/images/.img, while omitting sensitive data like user accounts.

If you need to, you can exclude some of your server files or directories from the generated image from /etc/ltsp/ltsp-update-image.excludes.

If for some reason the new image fails to boot, you can revert to the previous one using the Server > LTSP image > Revert menu.