Add basic constants to Color class

[dkrutsko]

Some useful predefined colors like Black, White and Transparent might be useful to add as constants to the Color class. Other constants such as the ones defined by QColor or XNA might also be useful but that might be too out of scope.

[zzztor]

I do not know how to implement it but probably this helps: http://www.adp-gmbh.ch/win/programming/graphic/color.html