Custom attributes should not be written by default

[thomlucc]

https://forum.babylonjs.com/t/how-can-i-remove-the-babylon-attributes-from-a-3dsmax-scene/22824/11

Also in Maya: #911

[pandaGaume]

@thomlucc , @bghgary
For the purpose, into Max first, I suggest to add a Menu Item within Babylon Menu to add/remove attribute such
Babylon->Material->CustomAttribute->Add, Remove, Remove all

• Add : Add a Babylon JS custom attribute to the selected material. Added attributes will be of different kind depending the selected node. ( PhysicalMaterial, Arnold, Standard, BumpMap, etc...)
• Remove : Remove a Babylon JS custom attribute from the selected material.
• Remove all : Remove Babylon JS custom attributes from all the scene materials. This lets user clean the scene from specific babylon attributes which can lead to issues when using the scene outside the Babylon exporter scope (see forum discussion on the topic)