[QBS] Group's property can not be accessed by qbs

[罗勇刚(Yonggang Luo)]

Sorry for the incorrect description, I means the following code can
not be evalulated:

Group {
    namespace: "goodDir"

    name: "source of " + namespace

    prefix: namespace + "/"

    qbs.install: project.installApiHeaders

    qbs.installDir: headerInstallPrefix + namespace //There is error here!
}

How to do things like this or just a bug in qbs?