[gtkmm] Container: Add a TODO comment about removing the virtual add().

[Murray Cumming <murrayc src gnome org>]

commit d95b163fd367cfb1cb397ed225aef9a33ada84a9
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Mar 5 11:33:17 2014 +0100

    Container: Add a TODO comment about removing the virtual add().
    
    See bug #724732.

 gtk/src/container.hg |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/gtk/src/container.hg b/gtk/src/container.hg
index 3cdce0e..94772fb 100644
--- a/gtk/src/container.hg
+++ b/gtk/src/container.hg
@@ -71,8 +71,10 @@ public:
   _WRAP_METHOD(guint get_border_width() const,
                gtk_container_get_border_width)
 
-  //This is virtual so that we can override it in Gtk::ScrolledWindow:
- virtual void add(Widget& widget);
+  //This is virtual so that we can override it in Gtk::ScrolledWindow.
+  //TODO: Remove the virtual keyword when we can break ABI,
+  //because the override in ScrolledWindow is no longer necessary.
+  virtual void add(Widget& widget);
   _IGNORE(gtk_container_add)
 
   _WRAP_METHOD_DOCS_ONLY(gtk_container_remove)