[longomatch] Make sure we clear selection before clearing objects.

[Andoni Morales Alastruey <amorales src gnome org>]

commit fda02ec1f07006e8a006c9f30ca2510a2a2c008d
Author: Julien Moutte <julien fluendo com>
Date:   Sat Mar 14 09:36:47 2015 +0100

    Make sure we clear selection before clearing objects.
    
    Indeed for a SelectionCanvas we could end up with invalid objects in the cached selection as ClearObjects 
does not clear the selection cache.

 LongoMatch.Drawing/Canvas.cs |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)
---
diff --git a/LongoMatch.Drawing/Canvas.cs b/LongoMatch.Drawing/Canvas.cs
index 316d851..3b05532 100644
--- a/LongoMatch.Drawing/Canvas.cs
+++ b/LongoMatch.Drawing/Canvas.cs
@@ -69,7 +69,7 @@ namespace LongoMatch.Drawing
                        }
                }
 
-               protected void ClearObjects ()
+               protected virtual void ClearObjects ()
                {
                        if (Objects != null) {
                                foreach (ICanvasObject co in Objects) {
@@ -180,6 +180,13 @@ namespace LongoMatch.Drawing
                        base.Dispose (disposing);
                }
 
+               protected override void ClearObjects ()
+               {
+                       // Make sure we don't maintain a selection with invalid objects.
+                       ClearSelection ();
+                       base.ClearObjects ();
+               }
+
                public int ClickRepeatMS {
                        get;
                        set;