Session: Update SessionNode when Project tree changes

This moves ownership of the project's root node from the
project into the project tree!

Change-Id: I84eba884bd63b44e56c75023d8bf12caf5cc2833
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/projectnodes.cpp

@@ -465,8 +465,6 @@ void FolderNode::buildTree(QList<FileNode *> &files, const Utils::FileName &over
                                                   overrideBaseDir);
         folder->addNode(fn);
     }
-
-    emitTreeChanged();
 }
 
 // "Compress" a tree of foldernodes such that foldernodes with exactly one foldernode as a child