Defines value filter. The value filters are supposed to take a
single value of a specific type and return a single value of
the same type. Filters are used to transforms values, e.g.
to clean up illegal characters, to do charset transcoding, etc.
Note:
Strictly speaking, filters should be used for things like
transliteration of national characters, case conversions, etc.
They should not be used to "technical" transformations such as
character set conversions. Connector should do that and connector
should return nice, clean unicode strings. But some connectors
are faulty and the ability of filters to do "technical" things
may come handy.