Support namespaced attached properties and singletons in qmlcompiler and qmlsc

Consider:

pragma Strict
import QtQml
import TestTypes as T

QtObject {
    id: self
    property date foo: self.T.BirthdayParty.rsvp
    property date bar: T.BirthdayParty.rsvp
    objectName: T.CppSingleton.objectName
}

All of this is legal, and lately QQmlJSTypePropagator can deal with it. However, it does not properly mark the import namespaces in the generated QQmlJSRegisterContent, and qmlsc crashes when trying to generate code for it.

qmlsc should generate some working code.