Make pre-defined property page non-movable

Fixed properties are fixed for a reason therefore prevent a property page that represents a pre-defined property to be non-movable.

Relates to #600.
>  I think this oddness is more of an issue than having the "and" in the hook handling function.

I understand and I will try to revisit this in future for a more notable solution but for now keeping this in this since it is not the worst and before adding something like `PropertyFactory` or similar there are some other oddities to clean-up before 2.1.

I will change the internal code flow to a more expressive `$this->isPropertyNamespace() ? $this->detectMovabilityForProperty() : true;` statement.