[gtkmm] Add a TODO comment

[Murray Cumming <murrayc src gnome org>]

commit cab6d33a1b466372f1e96d2edf4f47891f7f8e8f
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Apr 19 20:24:51 2011 +0200

    Add a TODO comment

 gtk/src/spinbutton.hg |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gtk/src/spinbutton.hg b/gtk/src/spinbutton.hg
index 0412957..edc4f38 100644
--- a/gtk/src/spinbutton.hg
+++ b/gtk/src/spinbutton.hg
@@ -50,6 +50,7 @@ class SpinButton : public Entry
   _IMPLEMENTS_INTERFACE(Editable)
 public:
 
+  //TODO: This constructor should be implicit
   //: create instance
   // adjustment: see Gtk::Adjustment
   // climb_rate: