Canvas: ensure all canvasses are destroyed

- For shutdown/reset make sure all properties describing a canvas
   are removed and also not restored after the reset.

simgear/canvas/Canvas.cxx

@@ -74,6 +74,12 @@ namespace canvas
     setStatusFlags(MISSING_SIZE_X | MISSING_SIZE_Y);
   }
 
+  //----------------------------------------------------------------------------
+  Canvas::~Canvas()
+  {
+
+  }
+
   //----------------------------------------------------------------------------
   void Canvas::onDestroy()
   {

simgear/canvas/Canvas.hxx

@@ -71,6 +71,7 @@ namespace canvas
       typedef osg::ref_ptr<CullCallback> CullCallbackPtr;
 
       Canvas(SGPropertyNode* node);
+      virtual ~Canvas();
       virtual void onDestroy();
 
       void setSystemAdapter(const SystemAdapterPtr& system_adapter);

simgear/props/PropertyBasedMgr.cxx

@@ -36,8 +36,8 @@ namespace simgear
   //----------------------------------------------------------------------------
   void PropertyBasedMgr::shutdown()
   {
+    _props->removeAllChildren();
     _props->removeChangeListener(this);
-    _elements.clear();
   }
 
   //----------------------------------------------------------------------------
@@ -96,7 +96,7 @@ namespace simgear
     _name_elements( name_elements ),
     _element_factory( element_factory )
   {
-
+    _props->setAttribute(SGPropertyNode::PRESERVE, true);
   }
 
   //----------------------------------------------------------------------------