... freeing three files from dependency cycles.
The class is only constructed by ODManager so move its definition there.
Meanwhile, the (now misnamed) ODTaskThread.cpp implements some thread
synchronization utilities used in several other places, not dependent now on
ODManager.cpp, and also not on ODTask.cpp.
These utilities should be eliminated in favor of standard library alternatives,
but that is a task for later.
... except Audacity.h
This forces us to make each header contain all forward declarations or nested
headers that it requires, rather than depend on context.