... Mostly into CommonCommandFlags.cpp, but some elsewhere, to avoid giving
that new file problematic dependencies on LabelTrack, ControlToolBar, and
EffectManager.
Note that CutCopyAvailableFlag is critically ordered, for message purposes,
only with AudioIONotAvailableFlag, the only flag with a message that it combines
with in menu item definitions.
The dependency on LabelTrack.cpp might not be a bad one later, if the track and
its view can be separated, and that would allow CutCopyAvailableFlag to be
put with the others. But much other work on LabelTrack must happen first.
... that is, a factory function, open, close, import, undo/redo/rollback.
Also the callbacks from AudioIO, which need to invoke undo history push when
recording stops.
It is meant as a high-level class using several of the other things attached
to the project, while AudacityProject will be a low level class acting mostly
as just the container of the attached structures.
... as a preparation for splitting up class AudacityProject.
Use ProjectWindow as an alias for AudacityProject, and fetch it from the
project with a static member function, where certain of its services are used;
pretending they are not the same class.
Use global accessor functions to get wxFrame from the project where only
wxFrame's member functions are needed, so there will be less dependency on
ProjectWindow when it becomes a distinct class.
... Generalizing what was first done at 280e8d9 for certain menu command
handlers.
Allow core data structures to host data attached by other code, on which it
need have no compilation or link dependencies.
