Support background colour in wxDataViewItemAttr in wxGTK

The code handling background colour was commented out for some reason, simply
enable it as it seems to be working just fine -- and update the sample to show
that it does.

docs/changes.txt

@@ -71,6 +71,7 @@ All (GUI):
 wxGTK:
 
 - Implement setting link colours in wxHyperlinkCtrl for GTK+3 (Hanmac).
+- Support background colour in wxDataViewCtrl attributes.
 
 wxMSW:
 

samples/dataview/mymodels.cpp

@@ -470,7 +470,8 @@ bool MyListModel::GetAttrByRow( unsigned int row, unsigned int col,
         case Col_IconText:
             if ( !(row % 2) )
                 return false;
-            attr.SetColour(*wxLIGHT_GREY);
+            attr.SetColour(*wxYELLOW);
+            attr.SetBackgroundColour(*wxLIGHT_GREY);
             break;
 
         case Col_TextWithAttr:

src/gtk/dataview.cpp

@@ -2248,7 +2248,6 @@ void GtkApplyAttr(GtkCellRendererText *renderer, const wxDataViewItemAttr& attr)
         g_value_unset( &gvalue );
     }
 
-#if 0
     if (attr.HasBackgroundColour())
     {
         wxColour colour = attr.GetBackgroundColour();
@@ -2268,7 +2267,6 @@ void GtkApplyAttr(GtkCellRendererText *renderer, const wxDataViewItemAttr& attr)
         g_object_set_property( G_OBJECT(renderer), "cell-background-set", &gvalue );
         g_value_unset( &gvalue );
     }
-#endif
 }
 
 } // anonymous namespace