Support a 'clear' operation on SGBinding.

Needed to bug-fix a problem with the menubar on Mac - post-3.0 I will
change the behaviour of SGBinding's destructor to avoid the property
copies in other use-cases.

simgear/structure/SGBinding.cxx

@@ -47,6 +47,14 @@ SGBinding::~SGBinding()
     _arg->getParent()->removeChild(_arg->getName(), _arg->getIndex(), false);
 }
 
+void
+SGBinding::clear()
+{
+    _command = NULL;
+    _arg.clear();
+    _setting.clear();
+}
+
 void
 SGBinding::read(const SGPropertyNode* node, SGPropertyNode* root)
 {
@@ -149,3 +157,11 @@ SGBindingList readBindingList(const simgear::PropertyList& aNodes, SGPropertyNod
     
     return result;
 }
+
+void clearBindingList(const SGBindingList& aBindings)
+{
+    BOOST_FOREACH(SGBinding_ptr b, aBindings) {
+        b->clear();
+    }
+}
+

simgear/structure/SGBinding.hxx

@@ -59,6 +59,14 @@ public:
   virtual ~SGBinding ();
 
   
+  /**
+   * clear internal state of the binding back to empty. This is useful
+   * if you don't want the 'remove on delete' behaviour of the 
+   * destructor.
+   */
+  void clear();
+
+
   /**
    * Get the command name.
    *
@@ -154,4 +162,9 @@ void fireBindingListWithOffset(const SGBindingList& aBindings, double offset, do
  */
 SGBindingList readBindingList(const simgear::PropertyList& aNodes, SGPropertyNode* aRoot);
 
+/**
+ * call clear() on every binding in a list
+ */
+void clearBindingList(const SGBindingList& aBindings);
+
 #endif