Enemy/Wave Spawning

Enemies can now be spawned in waves. At the moment wave 1 spawns 2
enemies and spawns an additional 4 each wave. Once all the enemies from
a wave are destroyed the Upgrade Menu appears. After buying something
the next wave starts.

Made the game start in the blue-print style by default since there are
no new graphics for the Ships or Row Boats yet (you can still press F2
to toggle between the new graphics and the blue-print style).

Fixed a bug that caused the Cannon to fire straight after you select an
upgrade in the Upgrade Menu.