Improvements to percussion

[peastman]

This makes lots of improvements to the percussion section.

• A lot of the samples for triangle and finger cymbals had noise in them. I cleaned them up.
• I added sampled releases to the snare drum and triangle rolls, which makes them sound cleaner.
• The wood block samples were missing from the All Percussion file. I added them.
• I combined the soft and loud bass drum sounds into a single note, using them as layers and cross fading between them.
• I combined the "left hand" and "right hand" hits for snare drum into a single note, using them as round robins.
• I renamed "All Percussion" to "All Unpitched Percussion", since that's what it is.
• There also was a file called "Various" which was just a subset of the sounds in "All Percussion". I deleted it since it didn't serve any purpose.