Ported window creation and input to OpenTK

This deprecates https://github.com/flibitijibibo/SDL2-CS and replaces it with @OpenTK. In practice this does not change much. The SDL2 backend is still preferred especially on Mac OS X, but it is not a hard dependency anymore. The GameWindow will fall back to other native implementations. This has several advantages:

* We gain portability to platforms where SDL2 is not supported or installed. Writing a renderer for OpenGL ES can then target mobile devices or Windows users without OpenGL capable drivers when ANGLE translates these to the DirectX API for us.
* The outdated MiniTK we used before has been dropped by upstream. https://github.com/flibitijibibo/SDL2-CS/issues/20 We now use one library that is actively maintained with tagged releases. This also makes life easier for Linux packagers when they unbundle our thirdparty dependencies. https://github.com/OpenRA/OpenRA/issues/6863
* Personally I like the C#yfied API. I don't need constructs like OpenGl.Gl.gl* to remind me of C namespaces/macros that look alien in the .NET world as I usually discover APIs through auto-complete in my IDE. This cleans up the code and allows even a newbie like me to work with these internals.
* I discovered that OpenTK can query display devices and change resolutions. This may help us resolve https://github.com/OpenRA/OpenRA/issues/6405 and https://github.com/OpenRA/OpenRA/issues/4440. Also https://github.com/OpenRA/OpenRA/issues/3774 may be possible with the OnResize events.
* The latest releases http://www.opentk.com/news/opentk-1.1.2-stable-released-with-cocoa-backend promise a lot of performance optimization in binding initialization and memory usage. I hope we benefit from it and from the improvements contributed by the OpenTK community.

There are also some disadvantages or known issues:

* I could not get PseudoFullscreen behavior to work so I removed it as also suggested in https://github.com/OpenRA/OpenRA/pull/6367.
* Clipboard access is not yet implemented so I had to remove it. http://www.opentk.com/node/3699#comment-15838
* The window will always grab the mouse. You can ALT+TAB to lose focus, but there is no free mouse cursor. I did not find any settings to emulate this old behavior.
* We lost support for some key codes. Most of them seem exotic though.
* The OpenTK.dll is quite big with 3.8 MB which gives us an estimated +2 MB for the release packages.

Also this requires extensive testing on all currently supported platforms for regressions. It will also be interesting if we already support *BSD now. https://github.com/OpenRA/OpenRA/issues/5446
How do I solve that best without adding redundancy?