Add support for il2cpp and iOS

[jsm174]

While trying to get Visual Pinball Engine running on iOS, I ran into following error:

VisualPinball.Engine.PinMAME.PinMameGamelogicEngine:OnInit(Player, TableApi, BallManager)
NotSupportedException: IL2CPP does not support marshaling delegates that point to instance methods to native code. The method we're attempting to marshal is: PinMame.PinMame::OnStateUpdatedCallback
  at PinMame.PinMameApi.PinmameSetConfig (PinMame.PinMameApi+PinmameConfig& config) [0x00000] in <00000000000000000000000000000000>:0 

After updating the callbacks to be static, another error occurred:

To marshal a managed method, please add an attribute named 'MonoPInvokeCallback' to the method definition

After reviewing discord/discord-rpc#186 I was able to implement MonoPInvokeCallbackAttribute.cs from this repo.

Todo:

• Better way of doing the static methods?
• Test in Windows + Mac.