Add: Customize groups for Ement faces

Each face now belongs to one of the following new groups: - `ement-room-faces` (sub-group of `ement-room`) - `ement-room-list-faces` (sub-group of `ement-room-list`) - `ement-tabulated-room-list-faces` (sub-group of `ement-tabulated-room-list`) Additionally, each of those new groups is also a sub-group of another new group `ement-faces` (which is parent to only the three 'faces' sub-groups). You can therefore navigate to each list of faces from either the customize group the faces used to belong to, or via the new `ement-faces` container. Also: - Delete duplicate defgroup ement-room-list. - Tweak phrasing for defgroup ement-tabulated-room-list for consistency with ement-room-list.