Ping changes
WaveController spawns graboids increasing per wave

Defers target selection to SurfaceModel, not yet implemented.
Placeholder currently defined in Game.
One type of city can be shown

4 populate the top of the screen under the dome. They can be
randomly targeted for attack.
Added custom event handling to Graboids

WaveController now associates an event listener to destroy
cities when they are hit by a graboid.