bgm intro+fade-in, easier finetuning in Inspector

--Added new clip to assets, it's an introduction clip for the music
--Tweaked the music playback script to accomodate the new clip
--Added a fade-in effect for the music on startup
--Created new public variables for laserScript and DemoBGMPlayer that
are visible in the Inspector:
in laserScript - sfxLaserChargeVolume and sfxLaserShootVolume
in demoBGMPlayer - fadeInRate
--Lowered both laser sfx volume slightly