Use libatspi to get the accessibility bus and handle main loop integration

Use libatspi  to get the accessibility bus and handle main loop integration

Remove function to get the accessibility bus, since it was duplicating code
in at-spi2-core and should be modified for BGO#644851.
Also, use atspi_dbus_*_setup_with_g_main instead of depending on dbus-glib,
so that FDO#35115 doesn't show up and we don't need to disable p2p if an old
version of dbus-glib is installed.

Note that at-spi2-atk now depends on libatspi where previously it did not.