We've started to turns containers into widgets which
just happen to have children, and some of these need
to be exposed to the a11y stack.
This adds a very minimal implementation, it does not
currently emit change notification when children are
added or removed.
This commit introduces GtkPicture, which is supposed to complement
GtkImage.
GtkImage will be adapted to always display an icon, while
GtkPicture displays regular imagery.
