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

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