Update foreword with all substantive changes made to the document

[hnlocher]

• Add change information in the foreword
  • Pixel Color Schemes for OPL processing are added (Annex C)
  • shim_id for image track files is modified and now include a complete URL with fragment part to accord to new other identifiers defined in the document (Table 7)
  • reference to SMPTE ST 2067-20 is suppressed, and SMPTE ST 2067-21 is now referenced to define Mastering Display elements (subclause 6.1.3.1.5)
  • the application identifiers changed and use new SMPTE namespace (subclause 7.1)

35PM-CD-ST-2067-40-revision2018-2020-07-28-11h53(redlineFrom2020-07-22)

[hnlocher]

Reflected in pull request #21