Implement MP3 Encoder

[eXpl0it3r]

With #1851 we implemented a MP3 decoder using minimp3, meaning you can open MP3 files and play them.

To fix the asymmetry, we should also implement a MP3 encoder, which allows to write MP3 files from a SFML sound buffer.

This issue has been create separately from #1232 to lower confusion between the more commonly understood "MP3 support", as in playing MP3 files, and feature symmetry, as in decoder & encoder.

[MarioLiebisch]

Stumbled over this again. Is there any library qualifying for this? Both LAME and Shine are LGPL, so would require yet another dependency (possibly loaded at runtime only?).

[ChrisThrasher]

Our one LGPL dependency OpenAL is a constant source of pain for users on Windows. I'd hope we can find a more permissively licensed option so that we can statically link it.