Rename namespace of code located under Source/Platform from WebKit to Nix.

The original name was inherited from Chromium times and created some
confusion on Nix upstreaming process requiring our explanation
that WebKit namespace doesn't refer to WebKit but to a public
API used for our platform abstraction.

Reviewed by: Luciano Wolf
Rename Nix::WebCanvas to Nix::Canvas.

Reviewed by: Marcelo Lira
Rename Nix::WebVector to Nix::Vector.

Reviewed by: Marcelo Lira
Rename Nix::WebSize to Nix::Size.

Reviewed by: Marcelo Lira
Rename Nix::WebRect to Nix::Rect.

Reviewed by: Marcelo Lira
Rename Nix::WebColor to Nix::Color.

Reviewed by: Marcelo Lira
Rename Nix::WebGamepad to Nix::Gamepad and Nix::WebGamepads to Nix::Gamepads.

Reviewed by: Marcelo Lira
Rename Nix::WebThemeEngine to Nix::ThemeEngine.

Reviewed by: Marcelo Lira
Change include guards of Platform.h to follow the same style of other files under Source/Platform/nix.

Reviewed by: Trust me.
Rename Nix::WebData to Nix::Data.

Reviewed by: Sergio Correia
Rename Nix::WebUChar to Nix::UChar.

Reviewed by: Thiago Lacerda
Rename Nix::WebAudioDevice to Nix::AudioDevice.

Reviewed by: Sergio Correia
Fixed cmake file to install Common.h instead of old WebCommon.h

Also renamed WebData.cpp to Data.cpp and fixed the related cmake file.

Reviewed by: Trust me.
Rename Nix::WebAudioBus to Nix::AudioBus.

Reviewed by: Thiago Lacerda
Rename Nix::WebFFTFrame to Nix::FFTFrame.

Reviewed by: Thiago Lacerda