Major CallbackDispatcher cleanup - Public facing API Changed

CallResult.SetAPICallHandle() has been renamed to CallResult.Set()
CallResult.ClearAPICallHandle() has been renamed to CallResult.Cancel()
CallResult.GetExpectedSize() has been renamed to CallResult.GetCallbackSizeBytes()
CallResult.GetExpectedCallback() has been renamed to CallResult.GetICallback()
CallResult.IsActive() has been added
The CallResult delegate functions have changed from:
 public delegate void APIDispatchDelegate(SteamAPICall_t callHandle, CallbackType param);
to:
  public delegate void APIDispatchDelegate(T param, bool bIOFailure);
These changes have been made to bring Steamworks.NET closer in line to
the official C++ Steamworks library.
The primary usage that these changes enable is the passing of bIOFailure
in CallResults to the delegate function so that client code can act
appropriately.