Use the functions in QRefCount to check if the QString is in use

QStringLiteral() returns a string that has a refcount of -1 and it must
be kept. There is no way to determine if it's still in use, so we'll
just assume it is. Any QStringLiteral inserted into the structure will
stay there forever.

On the other hand, we must not accept unsharable strings, so Q_ASSERT on
that.

Change-Id: I5fbdc1046f0f00319f27fdfb7aa3ff87371ea668
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>