a11y: Introduce an API for requesting speech from a screen reader
Sometimes, say, for some status changes communicated otherwise only visually, you want a way how to send a message to a screen reader.
This patch implements such API, along with a message priority modeled similarly to ARIA's live region politeness values.
We are intentionally not copying the AtkLive enum, as the value of ATK_NONE makes no sense for announcements.