introduce AsyncCommand for better handling of asynchronous operations

[lg2de]

Description

Some operations were defined asynchronously but not fully used like that.
Added custom AsyncCommand (inspired by https://docs.microsoft.com/de-de/archive/msdn-magazine/2014/april/async-programming-patterns-for-asynchronous-mvvm-applications-commands) triggering IsBusy for consistent progress information.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@307a78d). Click here to learn what that means.
The diff coverage is 69.14%.

@@            Coverage Diff            @@
##             master      #37   +/-   ##
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  Coverage          ?   84.75%           
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  Files             ?       37           
  Lines             ?     2808           
  Branches          ?      456           
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  Hits              ?     2380           
  Misses            ?      301           
  Partials          ?      127           

Impacted Files	Coverage Δ
...mpleAccounting/Infrastructure/ApplicationUpdate.cs	52.94% <0.00%> (ø)
...rc/SimpleAccounting/Infrastructure/RelayCommand.cs	75.00% <ø> (ø)
...pleAccounting/Infrastructure/SecureDriveStarter.cs	60.00% <ø> (ø)
.../SimpleAccounting/Presentation/AccountViewModel.cs	81.81% <ø> (ø)
...mpleAccounting/Presentation/AddBookingViewModel.cs	92.39% <ø> (ø)
...impleAccounting/Presentation/CloseYearViewModel.cs	93.75% <ø> (ø)
...Accounting/Presentation/ImportBookingsViewModel.cs	59.68% <ø> (ø)
...pleAccounting/Presentation/ImportEntryViewModel.cs	73.33% <ø> (ø)
...SimpleAccounting/Presentation/MessageBoxWrapper.cs	0.00% <0.00%> (ø)
...rc/SimpleAccounting/Presentation/ShellViewModel.cs	82.51% <62.50%> (ø)
... and 2 more

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