Overhauled BossType

- Config option for Piece of Power drop rate
- Updates to Skill Orb drops; Ending Blow added
- Totally rewrote BossType; now stored by name instead of id
- Biome lists for Boss Dungeons can now be configured
- Replaced magic numbers in ItemKeyBig
- Added boss type display name to big key names
- Renamed all boss key textures