examples: Add a GtkStack to the examples

Clicking on the "clickety" button in the gtk-clutter-test
will toggle, in a GtkStack, between a label, and a clutter-gtk
stage. It doesn't take very long to see the display artifacts after
that.

https://bugzilla.gnome.org/show_bug.cgi?id=695737
gtk-clutter-embed: Fix hiding embed inside container

Such as the embed inside a GtkStack, as used in Cheese or Totem.

With help from Matthias Clasen (positioning inside the window), and
Jasper St. Pierre (Wayland API help).

https://bugzilla.gnome.org/show_bug.cgi?id=736564