moved over flight generation logic

Demo:
-removed time_since_last_flight_generation
-removed generateFlight()
-removed createAircraft()
-reduced update()
+removed flight generation logic

GameWindow:
-added timeSinceFlightGeneration
-extended update()
+added flight generation logic
-added generateFlight()
-added createAircraft()
-modified getAvailableEntryPoints() result type