Make wxCOL_WIDTH_AUTOSIZE work correctly in Mac wxDataViewCtrl

Update the width when items are expanded and collapsed and also take the
expander width into account.

Change m_ModelNotifier type to avoid casts when calling wxOSX-specific
method on it.

Closes #14939.

Co-Authored-By: Vadim Zeitlin <vadim@wxwidgets.org>

include/wx/osx/cocoa/dataview.h

@@ -557,6 +557,9 @@ private:
     wxCocoaOutlineDataSource* m_DataSource;
 
     wxCocoaOutlineView* m_OutlineView;
+
+    // Width of expander in pixels, computed on demand.
+    int m_expanderWidth;
 };
 
 #endif // _WX_DATAVIEWCTRL_COCOOA_H_