Desktop User Notifications (Gnome, XFCE, KDE)

Based on this Discussion: https://askubuntu.com/a/473517
Tested with Ubuntu 20.04 (Gnome, XFCE and KDE)
Use Cases:
- You want to notify Desktop users after some package installation
- You want to notify Desktop users after doing some sysadmin tasks with Ansible or similar

Requirement

Usually comes pre-installed with Gnome and XFCE.
For KDE I had to manually install it.

apt install update-notifier-common

Files

All Desktops poll the directory /var/lib/update-notifier/user.d/ for changes.
KDE in shorter intervals than Gnome and XFCE.
It can take up to 5 minutes until the next poll comes around and the message gets displayed.

Gnome and XFCE display only new or changed messages since the last display.
KDE shows old messages again if they are still lying around.
So with KDE, make sure to remove old stuff in this folder
before putting new messages in it, so users don't get confused with old messages.

Examples

/var/lib/update-notifier/user.d/a-message

Name: A Message
Description: Hello Desktop

/var/lib/update-notifier/user.d/another-message

Name: Another Message
Description: Hello Desktop again

Gnome (A Message)

Gnome (Another Message)

Gnome (Both Messages)

Note the Next Button

XFCE (A Message)

XFCE (Both Messages)