Project: Add setRootProjectNode method

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>

src/plugins/cpptools/modelmanagertesthelper.h

@@ -50,7 +50,6 @@ public:
     ~TestProject() override;
 
     QString displayName() const override { return m_name; }
-    ProjectExplorer::ProjectNode *rootProjectNode() const override { return 0; }
 
     QStringList files(FilesMode fileMode) const override { Q_UNUSED(fileMode); return QStringList(); }