Efficient rendering of vector-based shapes and lines

- Stages replaced with EditorStage, in charge of a second ShapeRenderer pass
  (similar to the original SpriteBatch pass)
- Actors that wish to draw vector-based in this second pass must implement
  ShapeDrawable
- Replaced all "scaled empty boxes" with a 1 px border rectangle
- Fixes #148 (object-handles resizing themselves)
- Accesses Group's transforms to avoid additional transform matrix creation