Allow the creation of simple waveform sounds

`sf::SoundBuffer` can now be filled with standard waveforms.

This allows the creation of simple chiptune sounds on the fly using any of the following standard waveforms:

* Sawtooth
* Sine
* Square
* Triangle

It's possible to define the **length, frequency, sample rate, amplitude,** and optional **fade-in/fade-out** phases.

I felt like this functionality has been really lacking and it would also fit the general approach of SFML, considering it's also possible to create empty textures (without providing further means to manipulate them directly).

I've also created a [short Gist utilizing these changes](https://gist.github.com/MarioLiebisch/262e810c7944979abf22) to play children's songs encoded in a simplified ABC notation without the use of any additional files.

*Feel free to comment and nitpick as much as you can. I'll try to answer all questions and add corrections in case there are any issues. :)*