Support to export GLTF from View3D

Qt Quick3D should be able to export View3D scene and elements into GLTF. 

This is needed to be able to do 'OpenUSD to Qt'story. There are a lot of GLTF<>OpenUSD converters that can be used to work on the OpenUSD. OpenUSD is living protocol so keeping up with that is not relevant for us.

Exit criteria:

Import a GLTF scene (models, lights, materials, custom materials) from Blender and then return the project to Blender as export. Visuals can be different (different materials) but model geometry etc should be the same.