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.
11 jobs for wip/oholy/users-desktop-file in 3 minutes and 18 seconds (queued for 2 seconds)