Write metadata to export about blender3mfformat

[Ghostkeeper]

The collisions between different 3MF files has been resolved in the import stage (#11) and is no longer a concern.

The metadata between Blender's context and the 3MF file format is mapped as follows:

• Blender scene -> 3MF document
• Blender object -> 3MF item
• Blender mesh -> 3MF object resource

Only metadata that was written in the correct format in the Blender scene will get exported, for now. This prevents the add-on from writing custom properties that were set by other Blender add-ons. Writing those properties would also cause the add-on to import those custom properties again. This could mess with those other add-ons if the object properties were really specific to that instance of an object or perhaps only during that run of Blender. What's more, the importer would write the values of these properties as a dictionary format that is specific to this add-on, which is necessary to ensure that other attributes of the metadata entry get preserved as well (like the preserve and datatype attributes).

[Ghostkeeper]

This is completed with commit 9d2d54c and some preceding commits.