WebAudio API for Nix, using Chromium Platform APIs

This patch adds WebAudio Platform API for Nix
through Chromium's Platform APIs. This means that from now on
Nix won't be using its NixPlatform.h anymore. Hence, Nix::Platform namespace
and related files have been removed.

Nix will now rely on Chromium API types for its Platform layer. We have our
own Platform.h in Source/Platform/nix/public, so we can keep control of
the Platform APIs we support. The other headers (i.e. WebData, WebVector, etc)
we are using the Chromium ones directly.

This patch is turning GAMEPAD support off and turning WEB_AUDIO on by default.
For WEB_AUDIO an implementation for FFTFrame.cpp was needed, so we decided to use
FFTFrameFFMPEG for the time being. This pulled a new (and hopefully temporary
dependency to libavcodec).

GAMEPAD support will be re-enabled in a follow-up patch.

Reviewed by: Hugo Lima