CamelDataWrapper: Add construct_from_input_stream() method.

Takes a GInputStream instead of a CamelStream.  This will eventually
replace the construct_from_stream() method.

New functions:

  camel_data_wrapper_construct_from_input_stream_sync()
  camel_data_wrapper_construct_from_input_stream()
  camel_data_wrapper_construct_from_input_stream_finish()

Also adapted subclass methods in:

  CamelMimePart
  CamelMultipartSigned