TTS engines: use nicename

[marisademeglio]

from @bertfrees

and I have another one: the TTS engines have a nice name, e.g. the nice name of osx-speech is "macOS". This information can be found in the XML response from the server:

<tts-engines xmlns="http://www.daisy.org/ns/pipeline/data" href="http://localhost:8181/ws/tts-engines">
  <tts-engine name="osx-speech" nicename="macOS" status="available" voices="http://localhost:8181/ws/voices?engine=osx-speech"/>
  <tts-engine message="Property not set: org.daisy.pipeline.tts.google.apikey" name="google" nicename="Google" status="disabled"/>
  <tts-engine message="Property not set: org.daisy.pipeline.tts.azure.key" name="azure" nicename="Azure" status="disabled"/>
</tts-engines>