Install qbs_export.h.

It is referenced by other public headers and therefore has to be
installed as well.

Change-Id: If0973d36dd75e6822b1826f89884af7069003ea0
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Give the Project class a public constructor.

Otherwise API users cannot have it as e.g. a class member.
Also add the canonical isValid() method to check whether the object
was created by a SetupProjectJob.

Change-Id: I1deb995fa7f52bb8e2d58112b9386684b7c7eec8
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>