Refactor ShellViewModel and improve coverage

[lg2de]

Description

The ShellViewModel has received a lot of code not really intended for a view model.
These changes are focussed on refactoring and better test coverage.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a6eff59). Click here to learn what that means.
The diff coverage is 76.34%.

@@            Coverage Diff            @@
##             master      #36   +/-   ##
=========================================
  Coverage          ?   85.29%           
=========================================
  Files             ?       35           
  Lines             ?     2740           
  Branches          ?      436           
=========================================
  Hits              ?     2337           
  Misses            ?      276           
  Partials          ?      127           

Impacted Files	Coverage Δ
...rc/SimpleAccounting/Extensions/NumberExtensions.cs	100.00% <ø> (ø)
src/SimpleAccounting/Presentation/RelayCommand.cs	75.00% <ø> (ø)
...Accounting/Presentation/ImportBookingsViewModel.cs	59.68% <40.90%> (ø)
src/SimpleAccounting/Reports/XmlPrinter.cs	90.60% <46.15%> (ø)
...ting/Presentation/ImportBookingsDesignViewModel.cs	78.37% <50.00%> (ø)
...mpleAccounting/Infrastructure/ApplicationUpdate.cs	59.01% <59.01%> (ø)
...rc/SimpleAccounting/Presentation/ShellViewModel.cs	84.05% <72.22%> (ø)
...pleAccounting/Presentation/ImportEntryViewModel.cs	73.33% <83.33%> (ø)
...impleAccounting/Model/AccountingData.Extensions.cs	93.54% <94.62%> (ø)
...impleAccounting/Abstractions/SecureDriveStarter.cs	60.00% <100.00%> (ø)
... and 45 more

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