fix/playback_thread startup

if using a fallback the thread is started twice

fix test

fix test

missing import

Mycroft -> OpenVoiceOS

fix type hints

feat/tts_transformers

.

move PlaybackThread to ovos-audio

.

feat/ovos_dialog_tts_transformers

listener has AudioTransformers
core has MetadataTransformers and UtteranceTransformers

this PR extends the concept to ovos-audio for text before TTS stage, and wav file after TTS but before playback

use case demonstration https://gist.github.com/JarbasAl/16788bdcff3fa5bfb3634d6f2116cc04

this ties into OpenVoiceOS/ovos-persona#4

ovos_plugin_manager/templates/tts.py

@@ -327,7 +327,9 @@ def _init_playback(self, playback):
         TTS.playback.attach_tts(self)
         if not TTS.playback.enclosure:
             TTS.playback.enclosure = EnclosureAPI(self.bus)
-        TTS.playback.start()
+
+        if not TTS.playback._started:
+            TTS.playback.start()
 
     @property
     def enclosure(self):