New attached structure ProjectManager handles project lifetime...

... 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.

src/effects/EffectRack.cpp

@@ -297,7 +297,7 @@ void EffectRack::OnApply(wxCommandEvent & WXUNUSED(evt))
    auto state = UndoManager::Get( *project ).GetCurrentState();
    auto cleanup = finally( [&] {
       if(!success)
-         project->SetStateTo(state);
+         ProjectManager::Get( *project ).SetStateTo( state );
    } );
 
    for (size_t i = 0, cnt = mEffects.size(); i < cnt; i++)