Integrate to the Xfce config screen

[Xristh]

Actually mugshot is used on xubuntu and many other xfce4 based distros so it will be nice to have integration to the xfce4-config dialog, that integration can be achieved just adding

;X-XFCE-SettingsDialog;X-XFCE-PersonalSettings;

to the end of categories, so they will look like

Categories=GNOME;GTK;Settings;X-GNOME-Settings-Panel;X-GNOME-PersonalSettings;DesktopSettings;X-XFCE;X-XFCE-SettingsDialog;X-XFCE-PersonalSettings;

and that all, nothing more, just add that text and you are done.

[Sean Davis (bluesabre)]

Thanks for the suggestion! Applied at the below commit.

https://git.launchpad.net/mugshot/commit/?id=30695532f97341968b0d34c1cedd41e04814af8a