Modified cache API to include index and child count rather than children

The original cache API was problematic for QT AT-SPI because it forces
enumeration of all children, preventing lazy instantiation of objects.
The API now sends the object's index in parent and child count (or -1 if
not known / children should not be cached) rather than an array of
children.
Also made cache of children a GPtrArray rather than a GList, since it
may contain holes.  If an object has not yet been instantiated for a
particular child, then its value will be set to NULL, and
atspi_accessible_get_child_at_index will make a dbus call to fetch the
child, at which point it will be cached.

https://bugzilla.gnome.org/show_bug.cgi?id=650090