Refactor Notation code.

• Merge GpgME::UserID::Signature::Notation and GpgME::Signature::Notation into GpgME::Notation.
• Add signing notation handling on Context.
  • new function Context::clearSignatureNotations()
  • new function Context::addSignatureNotation()
  • new function Context::addSignaturePolicyURL()
  • new function Context::signaturePolicyURL()
  • new function Context::signatureNotation()
  • new function Context::signatureNotations()
• new function Context::signingKeys() const
• new function Signature::policyURL() const
• factor Error into definition into new error.h

svn path=/trunk/KDE/kdepimlibs/; revision=692878