a11y: Use a DOM-like API for iterating accessible objects

a11y: Use a DOM-like API for iterating accessible objects

The `get_child_at_index()` API model comes from AT-SPI, and it's not an
efficient design, especially when coupled with large widgets.

Replace `get_child_at_index()` with `get_first_accessible_child()` and
`get_next_accessible_sibling()`.

That allows efficiently retrieving all the children, simplifies the
implementation of GtkAccessible in GtkWidget and closely resembeles the
GtkWidget API.

Getting the last child and previous sibling for iterating backwards is
not a part of the interface at the moment, but they can be added at a
later date.

Note that this change required tracking the next stack page in
GtkStackPage.