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/commands/ImportExportCommands.cpp

@@ -41,7 +41,7 @@ void ImportCommand::PopulateOrExchange(ShuttleGui & S)
 }
 
 bool ImportCommand::Apply(const CommandContext & context){
-   return context.project.Import(mFileName);
+   return ProjectManager::Get( context.project ).Import(mFileName);
 }