Implementation of media player components.

Changed mediaControlsNix.css based on GTK.
Make RenderTheme to use Nix's custom style sheet for media elements.
Implementation of media mute button.
Implementation of media player background.
Implementation of media play button.
Implementation of media rewind button.
Implementation of media seekBack button.
Implementation of media seekForward button.
Implementation of media sliderThumb.
Implementation of media sliderTrack.
Implementation of media volumeSlider.

Patch by:
Lauro Neto <lauro.neto@openbossa.org>
Nick Diego Yamane <nick.yamane@openbossa.org>
Tullio Lucena <tullio.lucena@openbossa.org>
Add MediaPlayer Platform API to handle audio tag.

Video tag and WebAudio integration still not supported.

The feature is disabled by default while it's polished.

Reviewed by: Tullio Lucena