... so that USE_* macros will be seen first, and so
EXPERIMENTAL_AUTOMATED_INPUT_LEVEL_ADJUSTMENT, for one, is correctly defined.
I hesitated actually to include Audacity.h in Experimental.h. Maybe no
need for that hesitancy. But instead I use #error if Audacity.h has not been
seen yet.
... except Audacity.h; and in no others.
Do so even if Experimental.h gets multiply included, as in both the .h and
.cpp files.
This makes it easier to do a text scan to be sure there are no unintended quiet
changes of meaning because of omission of Experimental.h when the flag is
an enabled one.
Also move inclusions of Experimental.h earlier.
Also don't require Experimental.h to be preceded by Audacity.h to define
EXPERIMENTAL_MIDI_OUT correctly.
