Enhance filter API adding a FilterContext

As discussed oin the mailing list there are use cases where a PropertyFilter, similar to PropertyConverter requires additional (meta)data to be available, so it can implement the correct filtering. This can be solved by introducing a FilterContext. As a result the SPI is changed from

public String filter(String key, String valueToBeFiltered);

to

public String filter(String valueToBeFiltered, FilterContext context);