Improve accessibility of materials

Currently the Materials are parented below a seemingly random object that has the material assigned.

This works for very small scenes but may lead to longer searches for the material the bigger the scene gets. There are a couple of improvements that could be done to simplify finding/selecting materials:

1) Parent all materials under a dedicated set or group ("materials" or "looks") to have one spot to find the materials in question

2) Add a "Material Selection Mode" - when enabled and I select an object this mode could eventually select the material instead of the object.

3) Add a context menu to the "treeview" or "scene view" like "select assigned material" which selects the assigned material