qbs::Group is a reference-counted value-like class, and it makes no
sense to hold it as a pointer. It makes even less sense to compare that
pointer when deciding whether a project node needs an update, as this
comparison will always be false (and technically undefined behavior as
well if the original qbs::ProjectData object no longer exists).

Change-Id: If188f83802bf6cffd04215d192667cf0200e25d3
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>