Missing category for File Management capplet

[Luca Ferretti <elle uca libero it>]

After the recent switch to Control Center shell, the Nautilus
preferences capplet is listed under Others category.

To fix this we should simply add the proper category label for
Categories key in nautilus-file-management-properties.desktop.

Plausible categories are

        Personal (currently here are preferred applications, keyboard
        a11y, assistive technologies, keyboard shortcuts preferences)
        
        Look and Feel (windows, main menu, desktop bg, theme,
        screensaver, fonts preferences)
        
        System (?) (sabayon, pessulus, gnome-system-tools stuff, power
        manager, session properties)
        
Hardware and Internet & Networking are, of course, unrelated.

Tell me the preferred category, and I'll fix in on HEAD... hem, trunk.