Add C# generic Action CallOnMainThread goodness to Core that takes Action<PPError, T> and/or Action<PPError, T, U>
Add C# generic Action CallOnMainThread goodness to Core that takes Action<PPError, T> and/or Action<PPError, T, U>

- Allows to pass an Action with one or two user state objects on the callback when running within the CallOnMainThread.
- Makes it more C# oriented and strongly typed to Action.
- Cleaned up documentation and use C# 6.0 code.

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