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

Reviewed by: Nick Diego Yamane
Build fix in MediaPlayerPrivateNix::registerMediaEngine

Reviewed by: Sergio Correia
Gamepad id doesn't need to be UTF-16.

WebCore uses it as a UTF-9 string and Linux kernel reports it as a ASCII string.

Reviewed by: Nick Diego Yamane