at-spi2-core: upgrade 2.48.4 -> 2.50.0

at-spi2-core: upgrade 2.48.4 -> 2.50.0

Changelog:
===========
* Fix at-spi2-atk test when running under a non-English locale.
* collection: Avoid locking up if an object has a very large child count
* Fix possible NULL pointer dereference when deregistering an event listener.
* Various fixes for the new key grabbing API.
* Don't wait for a reply when registering or deregistering keystroke
  listeners.
* Add a meson option to exclude the gtk2 atk-bridge module.
* Fix a test failure/timeout.
* Add atspi_get_version() to return the runtime version of the library.
* Collection: fix match testing for attributes.
* Fix a NULL pointer dereference when deregistering an event listener that
  doesn't include a detail.
* Fix the documentation for interfaces in collection match rules.
* Add an AtspiLive enum to specify the politeness level for live regions,
  and document that this should be used for Announcement signals. Also add a
  notification signal in atk to replace the announcement signal that lacks
  the politeness information.
* Add an asynchronous version of atspi_generate_mouse_event.
* TableCell.xml: Add Get{Column,Row}HeaderCells methods
* atspi_accessible_get_localized_role_name: Use internal names when possible.
* Add a non-recursive function to clear a single object's cache.
* Code clean-ups.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>