MacroCommands remembers a reference to project...

... Eliminating several GetActiveProject calls, notably including those where
we construct a CommandContext for executing commands.

src/BatchCommands.h

@@ -87,7 +87,7 @@ class MacroCommands final {
       const FileExtension &extension);
    bool WriteMp3File( const wxString & Name, int bitrate );
    double GetEndTime();
-   static bool IsMono();
+   static bool IsMono( AudacityProject *project );
 
    // These commands do not depend on the command list.
    static void MigrateLegacyChains();
@@ -123,6 +123,9 @@ class MacroCommands final {
    void Split(const wxString & str, wxString & command, wxString & param);
    wxString Join(const wxString & command, const wxString & param);
 
+private:
+   AudacityProject &mProject;
+
    CommandIDs mCommandMacro;
    wxArrayString mParamsMacro;
    bool mAbort;