Transfer TreeDataGrid demo

[zdpcdt]

This pull request includes multiple changes primarily focused on renaming and reorganizing the project structure, as well as updating project references and adding new functionality for the TreeDataGridDemo page.

Project Structure and Organization:

• Removed the Semi.Avalonia.TreeDataGrid.Demo project from the solution file Semi.Avalonia.sln. [1] [2] [3]
• Renamed demo/Semi.Avalonia.TreeDataGrid.Demo to demo/Semi.Avalonia.Demo and updated namespaces accordingly. [1] [2] [3]

Project References and Dependencies:

• Updated demo/Semi.Avalonia.Demo/Semi.Avalonia.Demo.csproj to target .NET 8.0 and added a project reference to Semi.Avalonia.TreeDataGrid. [1] [2]

Functional Enhancements:

• Added new TreeDataGridDemo page with XAML and code-behind to demonstrate the usage of TreeDataGrid. [1] [2]
• Included a new style reference for TreeDataGrid in App.axaml.

ViewModel Updates:

• Updated FilesPageViewModel to use partial classes and observable properties, and refactored methods for better readability and functionality. [1] [2] [3] [4] [5] [6] [7] [8]

Data Enhancements:

• Added a static list of albums to the Song class in DataGridDemoViewModel.cs.

[Copilot]

Pull Request Overview

This PR reorganizes and renames the demo project for TreeDataGrid while updating project references, namespaces, and implementing new functionality for the TreeDataGridDemo page as well as enhancing viewmodel implementations.

• Removed the old Semi.Avalonia.TreeDataGrid.Demo project files and replaced them with the new Semi.Avalonia.Demo structure.
• Updated namespaces, project references (targeting .NET 8.0), and incorporated new C# syntax features (such as array and collection expressions).
• Added new view models and page code for demonstrating the TreeDataGrid functionality.

Reviewed Changes

Copilot reviewed 10 out of 19 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
demo/Semi.Avalonia.TreeDataGrid.Demo/ViewModels/SongsPageViewModel.cs	Removed as part of the reorganization.
demo/Semi.Avalonia.TreeDataGrid.Demo/Program.cs	Removed with the project structure update.
demo/Semi.Avalonia.TreeDataGrid.Demo/MainWindow.axaml.cs	Removed due to project reorganization.
demo/Semi.Avalonia.TreeDataGrid.Demo/App.axaml.cs	Removed with the updated project structure.
demo/Semi.Avalonia.Demo/ViewModels/TreeDataGridDemoViewModel.cs	Updated namespace and renamed main viewmodel class to reflect its role.
demo/Semi.Avalonia.Demo/ViewModels/TreeDataGridDemo/SongsPageViewModel.cs	New file to support the demonstration with a proper songs collection.
demo/Semi.Avalonia.Demo/ViewModels/TreeDataGridDemo/FilesPageViewModel.cs	Converted to partial class with ObservableProperty attributes and updated usage, including new C# syntax features.
demo/Semi.Avalonia.Demo/ViewModels/DataGridDemoViewModel.cs	Updated static album list using new collection expression syntax.
demo/Semi.Avalonia.Demo/Pages/TreeDataGridDemo.axaml.cs	Added page code to initialize the TreeDataGridDemo view with its viewmodel.
demo/Semi.Avalonia.Demo/Converters/FileIconConverter.cs	Updated namespace to reflect the new project structure.

Files not reviewed (9)

• Semi.Avalonia.sln: Language not supported
• demo/Semi.Avalonia.Demo/App.axaml: Language not supported
• demo/Semi.Avalonia.Demo/Pages/TreeDataGridDemo.axaml: Language not supported
• demo/Semi.Avalonia.Demo/Semi.Avalonia.Demo.csproj: Language not supported
• demo/Semi.Avalonia.Demo/Views/MainView.axaml: Language not supported
• demo/Semi.Avalonia.TreeDataGrid.Demo/App.axaml: Language not supported
• demo/Semi.Avalonia.TreeDataGrid.Demo/MainWindow.axaml: Language not supported
• demo/Semi.Avalonia.TreeDataGrid.Demo/Semi.Avalonia.TreeDataGrid.Demo.csproj: Language not supported
• demo/Semi.Avalonia.TreeDataGrid.Demo/app.manifest: Language not supported