nixos/gnome3: split up

This introduces the following options under the services.gnome3 namespace:

* core-os-services.enable
* core-shell.enable
* core-utilities.enable
* games.enable

The first three are all default enabled by gnome3.enable
and their purpose is to make gnome3 more flexable for users
usecases. In the case of core-utilities and games, it allows
users to easily switch on the default gnome3 applications
and games packages. Previously we had lists in gnome-3/default.nix
but they weren't visible to the user. By having options we have
generated documentation and an interface.