The current code keeps an instance of QbsProject and of qbs::Project in
every project node, with them being null/invalid everywhere except for
the top-level node. Instead, introduce a dedicated class for the root
node and hold a QbsProject only there. The qbs::Project is held in
QbsProject now.
As a nice side effect, this also fixes QBS-644, presumably because the
dubious-looking use of projectNode() has been removed.

Task-number: QBS-644
Change-Id: I5d36806745b9d67879db6f48aa56bc97868e4f17
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>