add a possibility to enable old gnome-2 preferences menu style with categories

BACK to 2008

For get this working the user have to be install /etc/xdg/menus/mate-preferences-categories.menu .
see
https://github.com/NiceandGently/mate-preferences-menus
In result the preferences menu looks like this
https://dl.dropboxusercontent.com/u/49862637/Mate-desktop/SOURCE/preferences-menu.png

Note:
If this file isn't installed on a system than you will see the normal menu.
This change doesn't break anything.
Maybe we can include this file in MATE as an extra package or as a installer option in mate-menus to create a sub package.