peak_amplitude_thread fix - #6807

peak_amplitude_thread fix - #6807

On windows AudioRegionEditor d'tor never returns, stalling or
crashing in pthread_join() after pthread_cancel().
Crashes happen if two thread pools with the same name exist.
(but since the thread has no event loop, no pool is needed)

After showing Region Properties any operation that would close the dialog
will either cause the application to hang indefinitely or terminate.

This also consolidates code: CrossThreadChannel works on windows these days