Preliminary function argument list changes and comments, for scrubbing project

In particular, use an options structure for AudioIO::StartStream to simplify
calls

ControlToolBar::PlayPlayRegion also takes that structure as an argument, and a
SelectedRegion instead of two times

And other changes

src/Project.h

@@ -82,6 +82,7 @@ class LyricsWindow;
 class MixerBoard;
 class MixerBoardFrame;
 
+struct AudioIOStartStreamOptions;
 
 AudacityProject *CreateNewAudacityProject();
 AUDACITY_DLL_API AudacityProject *GetActiveProject();
@@ -135,6 +136,8 @@ class AUDACITY_DLL_API AudacityProject:  public wxFrame,
                    const wxPoint & pos, const wxSize & size);
    virtual ~AudacityProject();
 
+   AudioIOStartStreamOptions GetDefaultPlayOptions();
+
    TrackList *GetTracks() { return mTracks; }
    UndoManager *GetUndoManager() { return &mUndoManager; }