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.
  This 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.
app: Keep props/wizard view in a Gtk.Box

Instead of putting sidebar and content_bin (container of wizard and
properties) in a clutter actor and relying on BoxLayout, lets just put
the whole thing in a Gtk.Box.
Drop now redundant Revealer class
notificationbar: Move it out of Clutter.Actor

We can use a Gtk.Overlay to display it.
collection-view: Derive from Gd.MainView

As per the new convention to derive widgets from gtk widget classes.
app: Use Gtk.Stack for below_bin

This saves us yet more actor layer.