refresh images
don't let missiles hit ghost UFO's
Add an Explosion sprite
remove superfluous codes, merge class files
update screens, add menu image
Rearrange game state logic to allow escaping to the menu, and resume a game in progress.
update menu image
hide in-game dialog when escaping to the menu
scenario wording update
adjust jet engaging timeout
ensure UFO tactical briefing words get drawn
add const.py
add a light fighter jet sprite
move out constants, add a new mode to fly a fighter jet.
update scenario 1 briefing
better jet fighter reload and firing times
scrolling tactical jet fight background
scenario updates
place dragging sprite on top
add some random parts from other levels