Move files for better organization

src/App.axaml.cs

@@ -0,0 +1,33 @@
+using System;
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Interactivity;
+using Avalonia.Markup.Xaml;
+using DialogHostAvalonia;
+using ReceiptPDFBuilder;
+using ReceiptPDFBuilder.ViewModels;
+
+namespace ReceiptPDFBuilder;
+
+public partial class App : Application
+{
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            desktop.MainWindow = new MainWindow();
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+
+    public void AboutOnClick(object? sender, EventArgs args)
+    {
+        DialogHost.Show(new AboutViewModel());
+    }
+}