TODO

- show ball count
- slider for max balls
- slider to scale damage
- slider to scale gravity
- slider for time scale
- add reset button
- add gravity between objects
- create a nice compact combo slider object w/ name and value
- reintroduce particles

Q: what to do in quadtree when we have more nodes at the exact same space than is
the maximum object count?
A1: we need to try splitting and check that we would not put all local objects into
the same new child quadrant.  If all children are destined for the same location,
then don't bother splitting and just increase the local size
A2: try splitting and if all children are destined for same child quad, then add
some random jitter until children go into separate child quads

Another thought is to use a dynamic splitting algorithm for the grid, the
object to grid allocations might not be retained over time quite as well as
a regular grid

how to support objects moving each turn?
1. for each object that has moved, find its current location and test if
it has left its quad.  If it has left its quad, then insert it into its parent.
2. rebuild quad-tree on every turn