XQuartz support (X11 on Mac)

This PR adds support for running OpenTK over XQuartz. XQuartz is an X11 implementation for Mac OS X.

End users are expected to use the native Cocoa backend on Mac. However, XQuartz support has the following advantages:
1. It uncovers Linux-specific code and improves portability to non-Linux X11 platforms (e.g. *BSD).
2. It simplifies debugging of X11-specific issues, such as https://github.com/mono/MonoGame/issues/2711
3. It proves the flexibility of the internal architecture of OpenTK, and highlights potential improvements.