Revamp the exporter UIs and add the missing requested export flags support

[sebavan]

For both Max/Maya, redesign the exporter UIs.

This should help addressing :

• Initial design exploration for feasibility
• GLTF exporter should be shared across max and maya.
• BackFace Culling add backface culling and double sided support in Maya #529
• Double Sided add backface culling and double sided support in Maya #529
• Transparency Mode Maya Exporter to support alphaMode MASK and alphaCutoff #535
• Max Simultaneous Lights Add exporter support for modifying BabylonMaterial.maxSimultaneousLights #501
• Custom Attributes Export custom Attributes from Maya #476
• Animation Only Animation Export only feature.  #525
• Add support for 3dsmax 2020 Why not support for 3dmax2020 until now? #618

All the above issues have been grouped here to simplify the development of the new UI.

Do not hesitate to add to the list.

[Drigax]

#566 has been checked in, decoupling the Babylon->GLTF exporter from the 3dsMax Babylon exporter implementation.

All that's left to do for 2) is to refactor Maya's exporter to use the shared implementation.