qdoc: Allow QML types that are not part of any QML module

QDoc expects QML types to be part of a QML module, i.e. that \inqmlmodule is set. Normally this is a valid assumption and it's good to have QDoc warn about a missing \inqmlmodule command, but in some cases we should allow QML type documentation that does not generate an import statement as one does not exist (e.g. if the type is available as a context property).