user-account: Use the same Categories as other panels
gnome-user-accounts-panel.desktop has some differences in "Categories" in comparison to other desktop files provided by gnome-control-center for no obvious reason. Add "GNOME" and "GTK" categories, which are used in all other desktop files. Remove "System" and use just "Settings" main category. This among others prevents the following output from desktop-file-validate: /usr/share/applications/gnome-user-accounts-panel.desktop: hint: value "System;Settings;X-GNOME-Settings-Panel;X-GNOME-DetailsSettings;" for key "Categories" in group "Desktop Entry" contains more than one main category; application might appear more than once in the application menu All other desktop files uses just the "Settings" main category. But maybe this is totally useless patch, because it seems that GNOME Shell do not care about the most of categories and GNOME Classic do not show those desktop files in menus at all.