Conversion of MonoMacGameView to OpenTK equivalent.

OpenTK for `MONOMAC` platform. Now MacOS is a peer to `WINDOWS` and `LINUX` platforms. Common code for desktop platforms.

I think it's ready for review / testing. RE: #2154.

**NOTES:**

* OpenTK controller mappings can replace SDL on other desktop platforms. Didn't consider that in scope for this PR.
* Media / Video needs some love. I need to compare how the other desktop platforms interface to `GameWindow` and try to emulate.
* Three intentional `#warning DISABLED DURING MONOMAC OPENTK CONVERSION` messages were added.
 * One in `OpenTKGameWindow.cs` to bypass icon extraction from assembly
 * One in `MacOS/GamerServices/Guide.cs` for `Window.ClearKeyCacheState()` which may not be needed now that we're using a different game loop.
 * One in `Media/VideoPlayer.MacOS.cs` for `_game.Window.AddSubview(_currentVideo.MovieView);`. That's the one that need some love.