Add core segment for stop events to speech memory

Requires #1 (closed) , #2 (closed)

Something like this:

Speech/
  SpeechToText/
    ...
  ...
  Event/
    <provider>/
      stop   # <- this is a defined entity name representing a stop event

Data type can be something simple, e.g. with string message and optional sourceID (e.g. SpeechToText snapshot).

Migrate MemoryStopListener from MycroftArmarX here