Log Message:
Update at-spi2-core to 2.20.0

* Support a stateless configuration by default (bgo#763540).
* Don't display warnings when connecting to an app that no longer exists.
* Don't display warning if unable to connect when logged in via ssh
  (bgo#761600).
* at-spi-bus-launcher: register with session manager (bgo#753931).
* Disable xevie by default--it probably doesn't do anything anyhow.
* get_index_in_parent: Don't crash if parent is defunct.
* Don't crash when trying to set an invalid state (bgo#757915).
* Don't crash if we get a children-changed event with a non-existent
  child (bgo#755951).
* atspi_hyperlink_get_index_range: don't return random values if the
  call fails (bgo#755727).
* Fixed some atspi_text_ functions (bgo#755731).
* Fixed crash during removal of last application in registryd (bgo#756513).
* Modified the cache API to specify an object's index and child count
  rather than its children. This eliminates the need for the application
  to enumerate its children, improving performance in some places with
  large lists (bgo#650090).
* Functions shouldn't try to return values (bgo#749330).
* Fix atspi_table_cell_get_position.