[glibmm] ArrayHandle: Add a comment about removing this one day.

[Murray Cumming <murrayc src gnome org>]

commit f4ba3c84bd87791cf41482e73f1908ab8d864326
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Jun 29 08:41:12 2015 +0200

    ArrayHandle: Add a comment about removing this one day.
    
    We still use it in lots of glibmm API, because glibmm (unlike gtkmm)
    hasn't had an ABI break since 2004.

 glib/glibmm/arrayhandle.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/glib/glibmm/arrayhandle.h b/glib/glibmm/arrayhandle.h
index 6938c64..87127eb 100644
--- a/glib/glibmm/arrayhandle.h
+++ b/glib/glibmm/arrayhandle.h
@@ -210,6 +210,10 @@ private:
 } // namespace Container_Helpers
 
 
+//TODO: When we can break ABI, remove this and replace uses of it with std::vector.
+//We cannot deprecate it yet, because we cannot easily deprecate methods that use it
+//- for instance, we cannot just override methods that use it as a return type.
+
 /** This is an intermediate type. When a method takes this, or returns this, you
  * should use a standard C++ container of your choice, such as std::list or
  * std::vector.