Add support for object.name attribute and Color Group in 3MF Materials Extension

[ansonl]

1. Added export support for retaining the object name in the object.name attribute that is supported for naming individual objects in the GUI of slicers such as Prusaslicer. Cura seems to export object.name set within Cura but not import existing object.name in files.

2. Added user toggle-able export support for writing material colors to Color Group/Color from the 3MF Materials and Properties Extension. Toggles between color group and base material. This supports 3MF viewers such as the online 3D preview on Printables.com that support Color Group to display objects in producer specified colors.

3. Updated README to remove TESTING category.

[CptJackieSparrow]

Can this version import color data to blender?