New material system

Goals:

• Materials as files
• Materials separated from the objects
• All objects should have reference materials
• Import materials from models as material files
• "Standard materials" and "Custom materials" both supported by the new filetype

Sub-tasks:

• Adding new materials in the editor
• Editing material files
• All new objects have reference materials that refer to a "Default" material
• Write and read material files when opening and editing materials in the editor
• Imported models should create material files, objects using said models should have reference materials pointing to those new material files
• UI work, editing materials, duplicating materials etc.
• Editing the material from the reference material

The system is to be implemented by having a hidden object inside the scene containing all the materials. Reference materials would then always refer to the objects inside that material container. By having a hidden object in the scene, no new system for editing materials not contained in the scene is needed. Reference materials also work only on objects inside the scene.

Problems:

• Materials have their own animations and properties and reference materials don't show any of these. => Reference materials should duplicate the behavior of the original materials in the timeline and inspector.
• Is the material container inside the master slide? => All slides would share the same material animations if said material is used.
• Alternatively, materials duplicated per slide?