Inform user that updates are running when trying to shutdown the system

 * Cinnamon version: 5.0.5
 * Distribution: Mint 20.2
 * Graphics hardware *and* driver used
 * 64 bit

Issue
When automatic updates are running and the user tries to shutdown the computer, the shutdown and restart buttons are just missing. This is especially confusing for end users who don't have technical insights (and those are the target group of the automatic updates feature, I configure automatic updates for them because I cannot require them to install updates manually). It would be good to show a small information text why the buttons are not visible.

Steps to reproduce
Enable automatic updates in Mint Update Manager, then wait until automatic update run starts or force it executing /usr/lib/linuxmint/mintUpdate/automatic_upgrades.py.

Expected behaviour
An information text should be displayed in the shutdown options window, that shutdown/restart is currently not possible because automatic updates are running.

Possible solution
Currently, the shutdown/reboot buttons are hidden by creating the file /etc/polkit-1/localauthority/90-mandatory.d/99-mintupdate-temporary.pkla from /usr/share/linuxmint/mintupdate/automation/99-mintupdate-temporary.pkla in automatic_upgrades.py.

In addition to that, the actual update script is executed using systemd-inhibit --why="Performing automatic updates" /usr/bin/mintupdate-cli. When opening the shutdown/reboot dialog, Cinnamon could check systemd-inhibit --list and display the message which was set by automatic_upgrades.py.

Start with the shutdown/reboot dialog in Cinnamon and the automatic update entry point /usr/lib/linuxmint/mintUpdate/automatic_upgrades.py. Review how systemd-inhibit --why="Performing automatic updates" /usr/bin/mintupdate-cli and systemd-inhibit --list expose the active reason. Done means the dialog explains that shutdown or restart is unavailable while automatic updates are running.