This introduces two new renderers used for sidebars: One for groups

(which recurses into subgroups), and one for items.
The renderers fit into the existing rb-c-sidebar styling. This has

been updated to support subgroups in the sidebar, which appear with a

bold (but not uppercase) header and nested items.
This in preparation for action registration for admin sidebar items,

which will allow extensions to augment the sidebar.

Unit tests pass.
Tested with an upcoming change converting the admin UI to the sidebar.