Add new ‘listener-safe’ SGProperty attribute

Used in FG to make certain tied properties as safe to use with
listeners, and hence suppress the warning.

simgear/props/props.cxx

@@ -849,7 +849,7 @@ SGPropertyNode::trace_read () const
  * Last used attribute
  * Update as needed when enum Attribute is changed
  */
-const int SGPropertyNode::LAST_USED_ATTRIBUTE = PROTECTED;
+const int SGPropertyNode::LAST_USED_ATTRIBUTE = LISTENER_SAFE;
 
 /**
  * Default constructor: always creates a root node.

simgear/props/props.hxx

@@ -821,6 +821,7 @@ public:
     USERARCHIVE = 64,
     PRESERVE = 128,
     PROTECTED   = 1 << 8,
+    LISTENER_SAFE = 1 << 9, /// it's safe to listen to this property, even if it's tied
     // beware: if you add another attribute here,
     // also update value of "LAST_USED_ATTRIBUTE".
   };