Add wxListCtrl::SetHeaderAttr()

This method can be used to change the list view header appearance.

Add the method declaration, documentation, show it in the sample and implement
it for wxMSW (only, for now).

interface/wx/listctrl.h

@@ -1052,6 +1052,25 @@ public:
     */
     bool SetColumnsOrder(const wxArrayInt& orders);
 
+    /**
+        Change the font and the colours used for the list control header.
+
+        This method can be used to change the appearance of the header shown by
+        the control in report mode (unless @c wxLC_NO_HEADER style is used).
+
+        Currently it is implemented only for wxMSW and does nothing in the
+        other ports.
+
+        @param attr The object containing the font and text and background
+            colours to use. It may be default, i.e. not specify any custom font
+            nor colours, to reset any previously set custom attribute.
+        @return @true if the attributes have been updated or @false if this is
+            not supported by the current platform.
+
+        @since 3.1.1
+    */
+    bool SetHeaderAttr(const wxItemAttr& attr);
+
     /**
         Sets the image list associated with the control.