Added basic SDL2 code for generating tones.
Starting to form an audio API.
Fleshed out the tone generation interface.
Play notes in sequence.
Renamed sound interface parts.
Abstracted away SDL in audio interface.
Replaced SDL_Sleep() invocation with standard C++ version.
Fleshing out the device interface.
Moved common SDL wrappers to their own file.
Moved the audio API from the experimental file.
Added #include guards for SDL header.
Added the synthesizer. Documentation to come.
Removed sound prototypes.
Simplified synthesizer implementation.
Merge branch 'dev' into sound
Added a type alias for audio volume.
Fixed indentation to reduce line width.
Started to document platform/audio.hpp.

- More to do.
Audio interface changes and documentation.
Added (fantasy) documentation for the synthesizer.