[Dev] Fix shell win+r too slow to appear/does not appear when triggered

[taooceros]

Shell plugin delegate of win+r is too slow to be a valid hook when triggered (we should largely adjust that part of code later), this pr add a Task.Run to run the slow part.

Also the Show method is not safe when triggered by threadpool, so the temporary fix is to wrap a Dispatcher.Invoke around it.

Also potentially fix sound effect sometimes disappear due to threadpool thread execute show method.

Test

• win+r works
• The window is at the right position
• Sound effect preserved

To reproduce an exception for the sound effect

1. Use a Task.Run wrap a IPublicAPI.ShowMainWindow()
2. Then you may notice that the sound disappear or an exception throw without the fix

Close #1584

[taooceros]

@gissehel may you give this pr a test? I think we may not need the dispatcher priority if execute everything in the main thread.

https://ci.appveyor.com/project/JohnTheGr8/flow-launcher/builds/45648485