qdoc: Fixed to report read-only QML properties correctly

qdoc: Fixed to report read-only QML properties correctly

Now the default for a QML property is writable. If qdoc
can't detect the actual read-only status, writable is
assumed. There were some cases where qdoc could not
determine the actual read-only/writable status for a
QML property. In these cases, qdoc reported read-only
because the default was read-only, which was not optimal.

Change-Id: I55aeb2bedcde92a414f4d48a8d995e5e9dbca5da
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>