For example, how to add a "Startup Applications Preferences" launcher to this Window?
Add custom App entries to System Settings
To add a custom app to System Settings (Gnome Control Center), find its desktop entry file -
After the modifications run this command:
This was tried and tested on Ubuntu 12.04 LTS12.04 with gnome-control-center version 3.4.2-0ubuntu0.12
Things to Note:
Right now adding custom items to System Settings is not support upstream from GNOME
Ubuntu patches System Settings
Long story short, you need to install
Right now there is no GUI method to add it. You need to do it programmatically using C and it would work only on Ubuntu.
The reasoning for not allowing third party entries (called panels) is detailed by David Zeuthen