Rewrite handling cell value changes in wxOSX/Cocoa wxDVC.

Instead of using a chain of dynamic_cast<>s to find the right type of the
value, construct a wxVariant corresponding to the type of the object we
receive in NSOutlineView:setObjectValue:forTableColumn:byItem and pass it to a
wxDataViewRenderer virtual function.

This fixes assert and allows to edit icon text items under OS X.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62436 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/dataview.h

@@ -82,6 +82,14 @@ public:
 
   void SetNativeData(wxDataViewRendererNativeData* newNativeDataPtr);
 
+
+#if wxOSX_USE_COCOA
+  // called when a value was edited by user
+  virtual void OSXOnCellChanged(const wxVariant& value,
+                                const wxDataViewItem& item,
+                                unsigned col);
+#endif // Cocoa
+
 private:
 //
 // variables
@@ -272,6 +280,13 @@ public:
 //
   virtual bool MacRender();
 
+#if wxOSX_USE_COCOA
+  // called when a value was edited by user
+  virtual void OSXOnCellChanged(const wxVariant& value,
+                                const wxDataViewItem& item,
+                                unsigned col);
+#endif // Cocoa
+
 protected:
 private:
     DECLARE_DYNAMIC_CLASS_NO_COPY(wxDataViewIconTextRenderer)