This patch enables spoken words without the need for external
festival using the existing FGVoiceMgr subsystem and the
properties under /sim/sound/voices
To hear the generated voices, set /sim/sound/voices/enabled=true
at startup
- use FLITEVoiceSynthesizer if a voice has <festival>false</festival>
- use FLITEVoiceSynthesizer if the festival server is unreachable
- decouple FGFLITEVoice from FGVoiceThread