fixed graphics bug with debris on Windows

Letting the alpha move into the negative range caused debris chunks to sometimes blink before disappearing.
prepared engine for sound support
added laser sound
added class SoundService and sound for explosions
modified window title
logic and rendering will be paused if the window is in background

also fixes the bug that the game reacts to keyboard/mouse input even if not in focus
spaceship can now only accelerate forward to make the game more challenging
removed unnecessary main menu reinits

The unnecessary reinits of the main menu caused bugs preventing to resume the game