[WIP] Wire up new mobile nav

References #3810

With GUI2, there's now a number of places to open the mobile navigation menu. This handles those clicks.

It's labeled as WIP, so please tear it apart.
Can we get a more descriptive action name, so that when I'm looking at this code buried deep, deep down in editor/edit/foo/bar and see it, I realize "Ah, this is the global sidebar toggle action". Don't know if 'nav' is the right word here, especially considering it's "sidebar" on your controller