Add an in-game editor

[psyGamer]

Adds a currently still very WIP in-game editor.

Current TODO list:

• [X] Actor placement/selection
• [x] Actor picker for placement
• [x] Tool system
• [ ] Local and global world spaces
• [ ] Geometry editing
  • [X] Vertex editing
  • [ ] Edge editing
  • [ ] Face editing
  • [ ] Vertex snapping
  • [ ] Edge snapping
  • [ ] Face snapping
  • [ ] Vertex colouring
  • [ ] Multi-texture fade
• [ ] Play testing spawn locations
  • [ ] Position you were last at
  • [ ] Checkpoint you last triggered
  • [ ] Cursor position from editor
  • [ ] Nearest checkpoint to cursor position from editor
  • [ ] A specific checkpoint
  • [ ] Start of level