GitLab

Add setRootProjectNode method and a default implementation of
rootProjectNode to Project. Use that in all derived classes.

Change-Id: Id28cde04457a20a8963d43020785ef9d77fea57c
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>