Add a "scene tree" viewer (Imgui)

[Adnn]

Should show the hierarchical structure of the scene (and its nodes) Selecting on the node should show the bounding box (allow debugging AABB)

• Dump names when available:
  • [x] nodes
  • [x] parts
  • [x] materials (material params from assimp + effect filename)
• [x] Handle default names
• [x] Make a root ImGui window to control others (via menu or checkboxes)
• Have subwindow showing details of a specifc node
  • [x] Parts (material, geometry info (vertex count, streams, ...))
  • [x] Nodes (transformations)