Add support for custom attributes to wxGTK wxDataViewCustomRenderer.

Call SetAttr() to store them in wxDataViewCustomRenderer before rendering it
and also honour the attributes in RenderText() (by reusing the same code we
already use for wxDataViewTextRenderer).

The attributes now work correctly in dataview sample under wxGTK as well.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62596 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/dvrenderer.h

@@ -58,6 +58,14 @@ public:
     // for details
     virtual bool GtkSupportsAttrs() const { return false; }
 
+    // if GtkSupportsAttrs() returns true, this function will be called to
+    // effectively set the attribute to use for rendering the next item
+    //
+    // it should return true if the attribute had any non-default properties
+    virtual bool GtkSetAttr(const wxDataViewItemAttr& WXUNUSED(attr))
+        { return false; }
+
+
     // these functions are only called if GtkSupportsAttrs() returns true and
     // are used to remember whether the renderer currently uses the default
     // attributes or if we changed (and not reset them)