[AT-SPI] Add child_bus attribute to parents having remote accessibility object child

[AT-SPI] Add child_bus attribute to parents having remote accessibility object child

This will allow AT-clients to query in a single dbus call (via collection iface) all busses used for UI embedding.
Result of such query may be used by AT-client to limit its dbus calls to:

a) active window process
b) processes that provide UIs embedded in active window

and avoid dbus calls to processes being in background, which respond very slowly.

Change-Id: Ifb73172cd63d67257ac9c03b9d98c3dbc23fe845