a11y: Publish API to update platform state
requested to merge michaelweghorn/gtk:michaelweghorn/a11y_publish_api_to_update_platform_state into main
API for updating the platform state was private so far, i.e. not available for custom GtkAccessible implementations.
This e.g. didn't allow custom implementations to update the focused state, which is essential for assistive technology to be able to track focus and announce the currently focused item.
Therefore, move the GtkAccessiblePlatformChange enum and the declaration of gtk_accessible_platform_changed to the public header.
Adjust gtk_at_spi_context_platform_change to not only work for GtkWidget, but any GtkAccessible.
Fixes: #6272