Use wxDataViewRenderer::PrepareForItem() in all ports

wxOSX and wxGTK previously used their own methods for handling the enabled
state and the attributes of the items being rendered, change them to reuse the
same methods as the generic implementation, i.e. SetEnabled() and SetAttr()
and remove the port-specific GtkSetAttr(), OSXApplyAttr() and so on.

This has the advantage of ensuring that the logic is the same for all
platforms (e.g. item enabled status wasn't handled in the same way in wxGTK as
in the other ports previously) and hopefully makes the code simpler by cutting
down on the number of virtual methods.

Notice that GtkSupportsAttrs() optimization was removed as it didn't seem to
be worth the bother (we basically saved a call to a virtual model method at a
price of a virtual renderer method call) and preserving it would have
complicated things needlessly.

include/wx/gtk/dvrenderer.h

@@ -55,37 +55,21 @@ public:
     void GtkInitHandlers();
     void GtkUpdateAlignment() { GtkApplyAlignment(m_renderer); }
 
-    // should be overridden to return true if the renderer supports properties
-    // corresponding to wxDataViewItemAttr field, see wxGtkTreeCellDataFunc()
-    // 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)
-    bool GtkIsUsingDefaultAttrs() const { return m_usingDefaultAttrs; }
-    void GtkSetUsingDefaultAttrs(bool def) { m_usingDefaultAttrs = def; }
-
     // return the text renderer used by this renderer for setting text cell
     // specific attributes: can return NULL if this renderer doesn't render any
     // text
     virtual GtkCellRendererText *GtkGetTextRenderer() const { return NULL; }
 
-
+private:
     // Change the mode at GTK level without touching m_mode, this is useful for
     // temporarily making the renderer insensitive but does mean that GetMode()
     // may return a value different from the actual GTK renderer mode.
     void GtkSetMode(wxDataViewCellMode mode);
 
 protected:
+    virtual void SetAttr(const wxDataViewItemAttr& attr) wxOVERRIDE;
+    virtual void SetEnabled(bool enabled) wxOVERRIDE;
+
     virtual void GtkOnCellChanged(const wxVariant& value,
                                   const wxDataViewItem& item,
                                   unsigned col);