Remove forgotten include directive from Nix::Size header.

Reviewed by: Sergio Correia
Remove #ifdef BUILDING_NIX__ hack from Nix::Rect header.

Reviewed by: Lauro Neto
Remove empty line :-)
Remove seek, maxTimeSeekable and setPlaybackRate from Nix::MediaPlayerClient API.

These methods belong to Nix::MediaPlayer API.

Reviewed by: Lauro Neto