Canvas: Don't crash on removing child hierarchie from map

Currently if a SGPropertyNode is removed from the property tree
only for this single node a childRemoved event is triggered. So
if we just check for a specific child to be removed, we will never
be notified. This commit manually triggers the event recursively
for all children but this should probably go directly into simgear.

src/Canvas/property_based_mgr.cxx

@@ -83,7 +83,9 @@ void PropertyBasedMgr::childAdded( SGPropertyNode * parent,
 void PropertyBasedMgr::childRemoved( SGPropertyNode * parent,
                                      SGPropertyNode * child )
 {
-  if( parent != _props || child->getNameString() != _name_elements )
+  if( parent != _props )
+    return canvas::triggerRemoveRecursive(child);
+  else if( child->getNameString() != _name_elements )
     return;
 
   size_t index = child->getIndex();

src/Canvas/property_helper.cxx

@@ -144,4 +144,15 @@ namespace canvas
     for( int i = 0; i < node->nChildren(); ++i )
       triggerChangeRecursive( node->getChild(i) );
   }
+
+  //----------------------------------------------------------------------------
+  void triggerRemoveRecursive(SGPropertyNode* node)
+  {
+    for( int i = 0; i < node->nChildren(); ++i )
+    {
+      SGPropertyNode* child = node->getChild(i);
+      node->fireChildRemoved( child );
+      triggerRemoveRecursive( child );
+    }
+  }
 }

src/Canvas/property_helper.hxx

@@ -98,6 +98,12 @@ namespace canvas
    */
   void triggerChangeRecursive(SGPropertyNode* node);
 
+  /**
+   * Trigger a childRemoved event for every child of node (Unlimited depth) and
+   * node itself.
+   */
+  void triggerRemoveRecursive(SGPropertyNode* node);
+
 } // namespace canvas
 
 #endif /* PROPERTY_HELPER_HXX_ */