changes following comments by STF160:

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          - import statements changed to avoid circular imports
            - import of LibSip_Common
            - types of LibSip_Common are commented out (Charstring_List, GenValue, GenericParam, SemicolonParam_List, AmpersandParam_List, CommaParam_List)
          - import statements changed to avoid dependency of PIXITs
            - import of LibSip_PIXITS commented out
            - c_mimeMultipart commented out
          - definition of DisplayName added
          - NameAddr changed to use DisplayName instead charstring (acc. to R5w140111)
          - featureCaps restored in MessageHeader
          - encoding rule "SIPCodec" added to types Request, *_Request, RequestUnion, Response (note: how to deal with this in the future is FFS)