version 0.98: minor fixes, redone levels

# enemies can _never_ move above or below visible screen
* "randomY"-enemies are placed entirely on screen (height calced)
# the major TI-OS crash bug WAS afterall caused by sprites drawn
   (partially) outside screen memory. temporarily fixed by setting
   virtual screen buffer to $8200 (enough mem there)
+ upto 29 cool enemy sprites and redone first five levels
* improved enemy-move routine; smooth luring, five speeds+backwards
# after pause weapon will not be fired
# teacher key fixed (waits for GRAPH to be release before&after)