Use Onshape PostMessage for selection UI

[eak24]

Right now the selection uses custom dropdown but should use https://onshape-public.github.io/docs/clientmessaging/ to make the part selections.

[thedunne]

Also I would recommend this sample code support assy. This example is going to be used by most 3rd party developers. The 2 areas where we need to "shore up" our knowledge transfer is, using the item selector, and the ability to successfully translate large data sets.

GLTF is now our "official" go to API for all partners who need to get tessellation information out of Onshape. So it makes this example very important.

3rd party devs will copy it verbatim. Add that this is a significant interest in an API "viewer".

Just makes this example this focal point for our API.

Joe