Redesigned the spawn system.
-FEATURE: Added Wall-waves.
-FEATURE: Added support for different wave types (randomised each time
it changes wave).
-FEATURE: Regular waves now spawn 3 times more enemies at the beginning
of a wave.
-OTHER: Changed pitch-scrolling in the score menu to only work if more
than 4 fingers are visible.
-CODE: Changed wave duration to depend on the wave type (through an
abstract hasFinished()-method in Wave).
-CODE: Added support for only allowing enemies to spawn in certain wave
types.
-CODE: Moved the update code for enemy spawns into the sub classes of
Wave.
-CODE: Added a Type-class in Wave to store the different wave types.
-CODE: Made Wave abstract and created a sub class for "regular" waves.
-CODE: Updated all Logger.log()-calls with the new parameter list.
-BUG: Fixed the problem with higher level enemies spawning in wave 1
after reset.