Convert DispatchEvent() calls to EventCallback #91

At present the DispatchEvent() calls return a void. It would be better for them to return a ValueTask as the methods they call are generally synchronous, but in some cases async.

I'll take a stab at doing this if @arnaudleclerc or one of the other core contributors both says it's a good idea and will review my changes. From @rbrundritt comment, I think this should work fine.