app: Remove a redundant actor
sidebar: Derive from Gtk.Revealer

And hence making Boxes.Revealer redundant.

It doesn't exactly work though:

* Container actor doesn't update its visibility based visibility of child.

* Doesn't expand vertically for some reason.

These will be fixed when we replace the containing actors with gtk
widgets in the following patches.
wizard,properties: Free them of actors

We still use an actor to contain the Gtk.Stack they are now in but even
that will be removed in a following patch.