Fixed bugs

*/ Saucer now check whether the target is on the scene before firing.
Lack of this check caused the game to crash

*/ Experimental - Added a little offset to the asteroids when they leave
the field to prevent them from jamming on the border of the scene

*/ Made the ship into a private data member of MainWindow class

*/ Moved the connection signal-slot from the MainWindow class to the
asteroid's and saucer's constructors

*/ Removed unnecessary boolean firedfromSaucer