Add ability to preserve transcript while using push to talk

This is useful when streaming. Occasionally the STT can get into
a bad state, and manually segmenting clears it up. However doing so
would clear your accumulated transcript, which isn't always desired. Add
ability to preserve the transcript.

A small wrinkle: the new commit logic requires N consecutive identical
windows before committing. To make this feature play nicely with it, I
had to forcibly commit any preview text that hasn't yet been committed.
Failing to do this would usually cause short utterances / the most
recently said stuff to get wiped out.

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