Remove unused stuff from RenderTheme.h.

Reviewed by: Thiago Lacerda
Put some love on Nix::Color class.

Summary:
1. Make Color class imutable.
2. Rename RGBA32 to ARGB32 because this is the internal representation of this type.
3. Remove default constructor, if you want a black or transparent color just use the
   statics Color::black and Color::transparent

Reviewers: lmoura, Tullio, luck, dakerfp, lacerda, setanta, sergio, nick

Differential Revision: http://phabricator.coderev.indt.org/D2979