Threadingbugsolution

[jspuij]

In addition to #88 I found another issue. The fact that the dispatcher was implemented as NullDispatcher (probably OK for single threaded Client Side Blazor), made it possible that user JSInterop calls from javascript would trigger concurrent renders on different threads.

I have implemented a proper dispatcher for the Synchronization context to schedule async calls on the render thread.

This pull request includes the code from the previous one. So merging this will probably suffice.