Fix crash in ProjectTree on restoring a session

On adding nodes to the node structure, both the FlatModel::nodesAdded
and ProjectTree::nodesAdded code is run. We need to ensure that
FlatModel::nodesAdded is run first though, as we need that information
in the ProjectTree.

Task-number: QTCREATORBUG-13667
Change-Id: I0b4b41ed6036cfdef668c16689d25611633ab0c9
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>