Make it possible to configure the visibility in the PropertyAssembler

[veewee]

Currently the PropertyAssembler generates protected properties by default.
Since we mostly don't extend these types, it is a better idea to change the default to private.

    /**
     * @var string
     */
    protected $someFilter;

This could be done by adding the visibility as a constructor parameter to the PropertyAssembler and changing the defaults in the Configuration file. This way you can still choose to use protected or even public if you're a real daredevil.

[veewee]

Fixed in #53