Add support for custom attributes to wxOSX/Cocoa wxDataViewCustomRenderer.

Virtualize applying the attributes in a new OSXApplyAttr() function and simply
store the attribute in its wxDataViewCustomRenderer version so that it could
be reused later in Render().

The attributes now work correctly in wxOSX/Cocoa dataview sample too.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62599 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/dvrenderer.h

@@ -83,6 +83,10 @@ public:
     virtual void OSXOnCellChanged(NSObject *value,
                                   const wxDataViewItem& item,
                                   unsigned col);
+
+    // called to ensure that the given attribute will be used for rendering the
+    // next cell (which had been already associated with this renderer before)
+    virtual void OSXApplyAttr(const wxDataViewItemAttr& attr);
 #endif // Cocoa
 
 private: