HOWTO Editing Gnome/KDE Menus

Introducion

You know the menu that's shown if you start the kde-menu, gnome-menu, xfce-menu or something else. There's no real name for this kind of menu (it's also often called "root-menu"), but the menu is saved in a Freedesktop-Standard. 

Where are the files saved?

The menus are made out of a "Main file" (.menu file). The file is written in XML, and it contains a kind of links to the Applications (.desktop files). 

The Main file detailing system-wide default menu structure might be located in one of the followings directorys: 
/etc/xdg/menus/ 
User-specific menu edits: ~/.config/menus/ 
If you have not found your file yet, search for it: find / -iname *.menu 

System-wide Menu entries (.desktop files) are stored in many places, for example: 
/usr/share/applications/ 
/usr/share/applink/
Gnome applications:
/usr/share/gnome/apps/
KDE applications: 
/usr/kde/3.5/share/applications/
User-specific applications: 
~/.local/share/applications/

Or search for the files: find / -iname *.desktop