Use major and minor version in the CSFML library names

[eXpl0it3r]

This solves two issues:

• For one it ensures that we're not using old or new versions of CSFML (e.g. Dispatch of KeyPress event throws a System.AccessViolationException #245) that may not be fully compatible
• It will also allow us to drop duplicated library versions in the CSFML nuget (no sfml-graphics.so or sfml-graphics.2.6.1.so).

[eXpl0it3r]

A limitation is, that it won't find default CSFML library names anymore, meaning on Linux you'd get libsfml-graphics.so.2.6, but now SFML.Net is looking for libsfml-graphics.2.6.so, nor for Windows where it's csfml-graphics.dll, it now needs to be csfml-graphics.2.6.dll.

I wonder if it's worth doing this hardcoding, or if we should just ship the ones needed. That would libsfml-*.so, libsfml-*.so.2.6, libsfml-*.dylib, and libsfml-*.2.6.dylib

[eXpl0it3r]

After the conclusions in #226, we won't go with this change.