Re-define blending of multiproperties

Changes compared to version 1.2.0 of this specification:

• blending of multiproperties has been re-defined
• the alpha-value of textures has been clarified
• displayproperties of multiproperties has been clarified.
• embedded example-3MF has been fixed
• links to the core specification refer to one of its released versions (currently, version 1.2.3)
• a number of intra-document links have been fixed

First public Markdown release; Realistic display properties

This is the first public release of the 3MF Materials and Properties Extension specification in Markdown.

Changes compared to version 1.1 of this specification:

• Realistic display properties have been added
• Multi-properties have been reworked
• RGB color blending has been standardized
• Formatting of this document has been changed to Markdown from the original docx-based version 1.1