project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/ViewModels/Popup.cs

@@ -0,0 +1,64 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading.Tasks;
+
+using Avalonia.Threading;
+
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace SourceGit.ViewModels
+{
+    public class Popup : ObservableValidator
+    {
+        public string HostPageId
+        {
+            get;
+            set;
+        }
+
+        public object View
+        {
+            get;
+            set;
+        }
+
+        public bool InProgress
+        {
+            get => _inProgress;
+            set => SetProperty(ref _inProgress, value);
+        }
+
+        public string ProgressDescription
+        {
+            get => _progressDescription;
+            set => SetProperty(ref _progressDescription, value);
+        }
+
+        [UnconditionalSuppressMessage("AssemblyLoadTrimming", "IL2026:RequiresUnreferencedCode")]
+        public bool Check()
+        {
+            if (HasErrors)
+                return false;
+            ValidateAllProperties();
+            return !HasErrors;
+        }
+
+        public virtual Task<bool> Sure()
+        {
+            return null;
+        }
+
+        protected void CallUIThread(Action action)
+        {
+            Dispatcher.UIThread.Invoke(action);
+        }
+
+        protected void SetProgressDescription(string description)
+        {
+            CallUIThread(() => ProgressDescription = description);
+        }
+
+        private bool _inProgress = false;
+        private string _progressDescription = string.Empty;
+    }
+}