Clean up dispatcher methods

[GeertvanHorrik]

There are several methods overlapping all and some returning void. All should return a task (which is up to the user whether it will be awaited or not).

Especially Dispatcherhelper.cs needs cleanup.

[GeertvanHorrik]

Need to correctly list all required InvokeAsync and BeginInvokeAsync methods. Also need to investigate what's available in .NET / WPF nowaways. Also need to keep similar API.

Items we should consider:

• InvokeIfRequired
• BeginInvokeIfRequired

I think the rest is now available as standard methods on the Dispatcher.