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.
P.S. Thanks for reviewing, @danzel. This is a major change, and I'd rather not be known as the jackass that broke MonoGame on MacOS. It's my primary dev environment, so I'm kicking the tires as well.

I'll add one more line comment to this PR to point out another area I think needs more scrutiny.