... This makes it impossible to forget to include the EXPERIMENTAL definitions
(such as when cutting and pasting code) and so get unintended quiet changes of
behavior.
The EXPERIMENTAL flags are now specified instead in new file Experimental.cmake
... 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.
... Should have no effect on generated code, except perhaps some slight faster
virtual function calls. Mostly useful as documentation of design intent.
Tried to mark every one of our classes that inherits from another, or is a
base for others, or has abstract virtual functions, and a few others besides.