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
Fix a compiler warning caused by commit 69e94310 for bug #724626
Bug #711436 - CalDAV: Ignore ctag on manual Refresh invocation
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