Orphaned files are now put back in the tree, or rejected if nowhere to put them

nw/core/tree.py

@@ -294,6 +294,11 @@ class NWTree():
                         tTree.append(tHandle)
         return tTree
 
+    def isValid(self, tHandle):
+        """Check if a handle exists in the project.
+        """
+        return tHandle in self._treeOrder
+
     ##
     #  Setters
     ##