Add a speechserver for Spiel

Spiel is a new a speech synthesis API for desktop Linux and others, allowing clients to easily perform TTS with a variety of synthesizers planned including eSpeak, Piper, and mimic3.

This is an initial implementation of the SpeechServer class, with support for basic screen reader functionality. A much better experience is possible with Spiel, as it matures alongside Orca's support.

closes #452 (closed)