Fix detectors, bullets, Add SpiralEmitterWeapon

-bullets will destroy themselves after fly across the detecting circle
-detectors will detect correctly even if some enemies die in the
detected region. Especially for MostForwardEnemyDetector.
-Add SpiralEmitterWeapon.
-WeaponDetector(and children): add GetCurrentTarget() for any use for
getting the target.
-Bullet: change the rotation to the direction, so the shape is not
limited to a circle. It can be some not symmetrical shapes now.
-Change the Enemy Buttons to "q" and "w" for saving the lives of "t" and
"R".