Fixing incompatibilities for new architecture

Airport:
-modified addToHangar()
+now takes time as a parameter rather than trying to retrieve from a
removes static function

AirportControlBox:
-added division_height to store height of a single division
+was recalculated multiple times; formulae now make more sense
+calculated in constructor
-removed dependency on window
+all references to window.height() in draw functions actually cancelled
themselves out...
+thse formulae now read much more clearly
-modified barProgress()
+now takes current_time as a parameter rather than trying to retrieve
from a removes static function

Altimeter:
-tidied comments
-deprecated isMouseOver()
+can only be calculated without parameters if it is placed on the base
coordinate system; it is not

GameWindow:
-updated to pass correct parameters to Airport.addToHangar()

Demo:
-removed imports that are no longer used since game logic moved to
GameWindow

DifficultySelect:
-now uses new location of game difficulty constants