Add header-bar notification revealer and statck init options

We used to just draw the current child to an offscreen pixbuf
when we switched to a new page, but that is not really correct
as it often happens that the child got queue_resize() on it
so the draw fails due to alloc_needed.

So, we completely change how we do this. In the case of an
animated transition we keep both the old and the new child
child_visible, then we expose the old child *once* in the normal
draw handler and then keep that pixmap around for the rest
of the transition.

That's the correct way to spell it.

Requires latest Gtk+ for special-case gtk_widget_set_opacity hack.

Instead of using the gtkwidget name which is theme related we use
a local property.

This also adds gd_stack_add_named helpers.

This is for clients who are interested in a passive notification when
the revealed/unrevealed status of the child widget has changed.

We want to add another read-only property which reflects the actual state
of the child (including the animation timeout), so rename this property first
for clarity.

This is not needed anymore since Documents doesn't use clutter-gtk.

This reverts commit 918bf2e7.

This makes the revealer works correctly when used with clutter-gtk.