Rename Base#_type property to #_class.
Derive Item#type from Item#_class and use that instead in code that depends on Item types.
Do not access internal _alpha property.
Simply call slice() instead of slice(0).
Fix typo.
First batch of large Color refactoring and simplification.

Work in progress.
Implement a row of simple optimizations to increase performance of Item constructors.
Increase performance of Style getters / setters.