Update 3

I've added basic level editor functionality. There's no proper UI yet, so press I toggle between input modes (aim, build, remove, move, rotate) and press O to toggle between buildable objects (wall, pickup, absorber, exit) in build mode. I recommend building in orthographic projection mode (toggle with P), because selecting built objects to remove/move/rotate them requires clicking it's "base", their sides or top won't detect clicks (unless their base happens to overlap them :). It's not possible to save levels yet, but stay tuned.