`ReflectionClass` allows reading of the values of private and protected
constants, and also to get private and protected static methods.
Therefore getting the values of private and protected static properties
is also permissible, especially since `::getStaticProperties()` already
allows to do so.
We also allow ::setStaticPropertyValue() to modify private and
protected properties, because otherwise this method is useless, as
modifying public properties can be done directly.
Christoph M. Becker