Added support for HSV, added convenience functions

Support for converting between RGB and HSV color spaces has been added.

Multiple convenience functions have been added to allow
setting/adding/subtracting from specific fields:

Setting fields: SetR, SetG, SetB, SetA
Adding to fields: AddR, AddG, AddB, AddA
Subtracting from fields: SubR, SubG, SubB, SubA

Changed ColorAlpha to use Color():SetA() internally.

Added Color():SetAlpha() as a wrapper for Color():SetA()