Add CamelFilterInputStream.

CamelFilterInputStream is similar to GConverterInputStream, except it
operates on a CamelMimeFilter instead of a GConverter.

This class is meant to be a temporary solution until all of Camel's MIME
filters are ported to the GConverter interface.
Add CamelFilterOutputStream.

CamelFilterOutputStream is similar to GConverterOutputStream, except it
operates on a CamelMimeFilter instead of a GConverter.

This class is meant to be a temporary solution until all of Camel's MIME
filters are ported to the GConverter interface.
Camel: Port mime-filter tests to use GIO streams.