Add custom App entries to System Settings

To add a custom app to System Settings (Gnome Control Center), find its desktop entry file - /usr/share/applications/{appname}.desktop and edit it to ensure that the following entries are present:

Categories=Settings;X-GNOME-Settings-Panel;xxxxx;

X-GNOME-Settings-Panel={appname}

Replace {appname} with the name of the App you're adding to System Settings.
xxxxx decides the Section where your app is displayed. Replace xxxxx with one of these possible options:

X-GNOME-PersonalSettings - Personal Section

HardwareSettings - Hardware Section (Note that prefix X-GNOME is not used)

X-GNOME-SystemSettings - System Section

After the modifications run this command:

sudo update-desktop-database

This was tried and tested on Ubuntu 12.04 LTS12.04 with gnome-control-center version 3.4.2-0ubuntu0.12

Things to Note:

1. System Settings don't recognise desktop files in
   ~/.local/share/applications and as a result,it's not possible to
   add Apps to System Settings for one user alone.


2. As noted by Manish Sinha in this Answer, Ubuntu uses a
   gnome-control-center with Ubuntu specific patches which enables the
   addition of custom entries. As a result,this method is Ubuntu specific and won't work on vanilla gnome-control-center used by other distros.