Improve purchase editing

[VMelnalksnis]

Fix bug introduced in #371.

Changes proposed in this pull request:

• Add modal dialog for product creation for purchases Easier way to create a new product while editing a transaction #369
• Initialize delivery date to the last entered value in the current transaction

[codecov]

Codecov Report

Merging #372 (52ff777) into master (6fb0ba3) will decrease coverage by 0.11%.
The diff coverage is 57.50%.

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
- Coverage   67.45%   67.34%   -0.12%     
==========================================
  Files         247      248       +1     
  Lines        6907     6926      +19     
  Branches      487      500      +13     
==========================================
+ Hits         4659     4664       +5     
- Misses       2087     2097      +10     
- Partials      161      165       +4     

Impacted Files	Coverage Δ
...valonia.Core/DesignTime/DesignTimeDialogService.cs	0.00% <0.00%> (ø)
...de.Interfaces.Avalonia.Core/MainWindowViewModel.cs	17.14% <0.00%> (-0.13%)	⬇️
...ansactions/Purchases/PurchaseUpsertionViewModel.cs	79.01% <11.11%> (-7.10%)	⬇️
...a.Core/Transactions/Purchases/PurchaseViewModel.cs	69.51% <62.50%> (-1.92%)	⬇️
...erfaces.Avalonia.Core/DesignTime/DesignTimeData.cs	99.13% <100.00%> (-0.05%)	⬇️
...Core/Transactions/TransactionUpsertionViewModel.cs	64.19% <100.00%> (+0.44%)	⬆️
...Avalonia.Core/Transactions/TransactionViewModel.cs	72.05% <100.00%> (+0.41%)	⬆️
...ade.TestingHelpers/Data/Fakers/NamedEntityFaker.cs	57.14% <0.00%> (-14.29%)	⬇️
...ia.Core/Transactions/Purchases/PurchaseOverview.cs	86.95% <0.00%> (+4.34%)	⬆️

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