[gtkmm] LevelBar: Document how to workaround the lack of Orientation base class.

[Murray Cumming <murrayc src gnome org>]

commit f3229bc34d9a4bbbbff2000302032e1c9227ffe3
Author: Murray Cumming <murrayc murrayc com>
Date:   Sat Jul 18 21:52:47 2015 +0200

    LevelBar: Document how to workaround the lack of Orientation base class.
    
    Bug #752568 (Tobias Hoffmann)

 gtk/src/levelbar.hg |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)
---
diff --git a/gtk/src/levelbar.hg b/gtk/src/levelbar.hg
index 07bfdda..93a7a18 100644
--- a/gtk/src/levelbar.hg
+++ b/gtk/src/levelbar.hg
@@ -26,6 +26,9 @@ namespace Gtk
 
 _WRAP_ENUM(LevelBarMode, GtkLevelBarMode)
 
+//TODO: Derive/Implement from Gtk::Orientable (and remove our mention of that
+//in the class documentation when we can break ABI.
+
 /** A bar that can used as a level indicator
  *
  * Typical use cases are displaying the strength
@@ -50,6 +53,13 @@ _WRAP_ENUM(LevelBarMode, GtkLevelBarMode)
  * set the minimum value to 0 and the maximum value to 5 after changing the indicator
  * mode to discrete.
  *
+ * gtkmm's LevelBar class should derive from the Orientable interface but that would
+ * now require an ABI break. You may instead use the C API to change the orientation.
+ * For instance:
+ * @code
+ * gtk_orientable_set_orientation(GTK_ORIENTABLE(bar.gobj()), GTK_ORIENTATION_VERTICAL);
+ * @endcode
+ *
  * @ingroup Widgets
  * @newin{3,6}
  */