a11y: support GetAccessibleId for at-spi
This introduces a new helper function accessibleIdForAccessible (inspired by Windows' automationIdForAccessible) to synthesize an id out of the objectNames of the accessible parent chain. The id is then exposed via the GetAccessibleId D-Bus function for consumption in a11y tools.
Change-Id: If72b86c5864c43f4ca842aa11423dd8aea0dde4a Reviewed-by: Aleix Pol Gonzalez aleixpol@kde.org (cherry picked from commit 75e87548)