POC: toggle extrusions and travel moves without re-rendering everyting

[sophiedeziel]

Part of #282

I extracted an object manager. That god class we had was reduced to demigod in #309 and now it's just a nice and reasonable class! sub 900 lines and next commits will reduce it even more!

What that manager does is to keep track of the meshes, materials and stuff. When it's time to dispose of them, it disposes of them. When we just want to hide them or render them again, we don't need to re-calculate everything from scratch.

WIP

• Tests are failing because I did not update them
• I want to also move the build volume and bounding box
• The scene manager's init, reset, clear and dispose need some attention. It's not 100% right.