Sidebar has no hook

Currently, the sidebar cannot be modified / added to by a plugin. I already have a small patch (adding 16 lines, deleting and modifying none in two core files) for a hook that allows creating a plugin that would extend the sidebar.

Would a PR be considered / reviewed? It would come with several tests to assert that the sidebar is not changed if the hook is not used.

I would, of course, be open to modification / extension requests from your side.