version 0.92: fix regressions, minor updates

+ all bullets and enemies are removed at the start of a level
# you can fire bullets again (no more laser at startup)
* pickups aren't given at random, but appear every x enemies destroyed
# solved a minor bug in invulnerability flashing display
* clearScreen procedure improved (faster and smaller)
# fifth icon couldn't be selected (so no multiples selectable). fixed
* laserbeam lasts 5 turns (laser wasnt good enough compared 2 bullets)