Warning cleanup

This fixes quite a few of the warnings in the current build.

A lot of them are caused by bad comments.
Some by changes to OpenTK.
All the iOS changes are deprecated APIs.

There are still more warnings. We will see what the build bot has to say!

I know that iOS has iOSGamePlatform.IsPlayingVideo which is marked as Obsolete, and causes a lot of warnings.
This has the comment "iOSGamePlatform.IsPlayingVideo must be removed when MonoGame fully implements the XNA VideoPlayer contract.".

Also it uses GKPeerConnectionState which is deprecated in iOS7, but it doesn't look like we can remove it without a fair bit of work.

There are also more warnings in the Content Processor files I haven't touched. Mostly fields that are never set. I'm not sure if these are required for marshalling or anything along those lines, so I've left them.

refs #2433