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/dvrenderers.h

@@ -30,6 +30,8 @@ public:
 
     virtual bool MacRender();
 
+    virtual void OSXApplyAttr(const wxDataViewItemAttr& attr);
+
     virtual wxDC* GetDC(); // creates a device context and keeps it
     void SetDC(wxDC* newDCPtr); // this method takes ownership of the pointer