Generates a polygonal border for newly-created sceneElements

- this detailed border is shown only when object is selected
- the detailed border is used for collision detection
- lowered default threshold for border-detection

NOTE:
  if more uses of ShapeRenderer arise, we should use a similar
  approach to Drawable to pool these renderers. See
  https://github.com/libgdx/libgdx/issues/1186