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/dataview.h

@@ -166,6 +166,8 @@ public:
     bool GetBold() const { return m_bold; }
     bool GetItalic() const { return m_italic; }
 
+    bool IsDefault() const { return !(HasColour() || HasFont()); }
+
 private:
     wxColour m_colour;
     bool     m_bold;