Don't begin global paths with '/' on windows.

Doing it creates huge delays, because it resolves to a network path.
Also rename FolderNode::setFolderName to FolderNode::setDisplayName,
because that's the name of the property.

Task-number: QTCREATORBUG-695

src/plugins/projectexplorer/session.cpp

@@ -359,7 +359,7 @@ void Internal::SessionNodeImpl::removeProjectNode(ProjectNode *projectNode)
 void Internal::SessionNodeImpl::setFileName(const QString &fileName)
 {
     setPath(fileName);
-    setFolderName(fileName);
+    setDisplayName(fileName);
 }
 
 /* --------------------------------- */