Add Enemy2 class, a tougher Enemy class

Also changed enemy generation rate to use a Poisson-distributed random
number generator.
Add temporary images to Enemy and Enemy2
Revise container hierarchy to correct z order of objects

Previously, there have been two types of object having problems:

* Explosion effects are drawn topmost, hiding score board and
  buy buttons.
* Tower status tip is contained by the tower shape object, which
  can be blocked by other towers.

We use intermediate containers to fix those problems.

Tower status tip is no longer child of Tower graphic object itself.
It's now on overlayTip container.  We have to manually adjust position
of the tip to follow the graphic in this way.
Add tip hint to delete button to explain it usage