Don't untie a property when loading from xml

executing the fgcommand loadxml implicitely untied every previously
tied property if a property type was given in the property element.
That made it impossible to autosave aircraft data for tied properties.
This patch should fix this. It adds an inconsitency, though:
if a property with a given type is read in which has been tied to
before and the property types do not match, the property type in the
xml file will be ignored.

simgear/props/props_io.cxx

@@ -262,7 +262,10 @@ PropsVisitor::startElement (const char * name, const XMLAttributes &atts)
     }
 
     const char *type = atts.getValue("type");
-    if (type)
+    // if a type is given and the node is tied,
+    // don't clear the value because
+    // clearValue() unties the property
+    if (type && false == node->isTied() )
       node->clearValue();
     push_state(node, type, mode, omit);
   }