Add possibility to set item background in generic wxDataViewCtrl.

Added wxDataViewItemAttr::SetBackgroundColour() and code to honour it in the
generic implementation of wxDataViewCtrl.

Closes #12621.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70050 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/dataview.h

@@ -580,6 +580,16 @@ public:
     */
     void SetColour(const wxColour& colour);
 
+    /**
+        Call this to set the background colour to use.
+
+        Currently this attribute is only supported in the generic version of
+        wxDataViewCtrl and ignored by the native GTK+ and OS X implementations.
+
+        @since 2.9.4
+    */
+    void SetBackgroundColour(const wxColour& colour);
+
     /**
         Call this to indicate that the item shall be displayed in italic text.
     */