Added view models and storage providers.

Harmonia.WinUI/Storage/FilePickerFileType.cs

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+using System.Collections.Generic;
+
+namespace Harmonia.WinUI.Storage;
+
+public sealed class FilePickerFileType
+{
+    public string Name { get; set; } = string.Empty;
+    public IReadOnlyList<string> Patterns { get; set; } = [];
+}

Harmonia.WinUI/Storage/FilePickerOptions.cs

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+using System.Collections.Generic;
+
+namespace Harmonia.WinUI.Storage;
+
+public class FilePickerOptions
+{
+    public IReadOnlyList<FilePickerFileType> FileTypeFilter { get; set; } = [];
+    public StoragePickerViewMode ViewMode { get; set; }
+}

Harmonia.WinUI/Storage/FolderPickerOptions.cs

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+namespace Harmonia.WinUI.Storage;
+
+public class FolderPickerOptions
+{
+    public StoragePickerViewMode ViewMode { get; set; }
+}

Harmonia.WinUI/Storage/IStorageProvider.cs

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+using System.Threading.Tasks;
+
+namespace Harmonia.WinUI.Storage;
+
+public interface IStorageProvider
+{
+    public Task<string> GetFileAsync(FilePickerOptions? options = null);
+    public Task<string[]> GetFilesAsync(FilePickerOptions? options = null);
+    public Task<string> GetPathAsync();
+}

Harmonia.WinUI/Storage/StoragePickerViewMode.cs

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+namespace Harmonia.WinUI.Storage;
+
+public enum StoragePickerViewMode
+{
+    List,
+    Thumbnail
+}

Harmonia.WinUI/Storage/WindowsStorageProvider.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Windows.Storage;
+using Windows.Storage.Pickers;
+
+namespace Harmonia.WinUI.Storage;
+
+public class WindowsStorageProvider(MainWindow mainWindow) : IStorageProvider
+{
+    public async Task<string> GetFileAsync(FilePickerOptions? options = null)
+    {
+        FileOpenPicker fileOpenPicker = GetFileOpenPicker(options);
+        InitializePicker(fileOpenPicker);
+
+        StorageFile storageFile = await fileOpenPicker.PickSingleFileAsync();
+
+        return storageFile.Path;
+    }
+
+    public async Task<string[]> GetFilesAsync(FilePickerOptions? options = null)
+    {
+        FileOpenPicker fileOpenPicker = GetFileOpenPicker(options);
+        InitializePicker(fileOpenPicker);
+
+        var storageFiles = await fileOpenPicker.PickMultipleFilesAsync();
+
+        return [.. storageFiles.Select(storageFile => storageFile.Path)];
+    }
+
+    private static FileOpenPicker GetFileOpenPicker(FilePickerOptions? options = null)
+    {
+        FilePickerOptions filePickerOptions = options ?? new();
+
+        FileOpenPicker fileOpenPicker = new()
+        {
+            ViewMode = GetViewMode(filePickerOptions.ViewMode)
+        };
+
+        string[] filters = GetFilters(filePickerOptions.FileTypeFilter);
+
+        foreach (string filter in filters)
+        {
+            fileOpenPicker.FileTypeFilter.Add(filter);
+        }
+
+        if (filePickerOptions.FileTypeFilter.Count == 0)
+        {
+            fileOpenPicker.FileTypeFilter.Add("*");
+        }
+
+        return fileOpenPicker;
+    }
+
+    private static PickerViewMode GetViewMode(StoragePickerViewMode viewMode)
+    {
+        return viewMode switch
+        {
+            StoragePickerViewMode.Thumbnail => PickerViewMode.Thumbnail,
+            _ => PickerViewMode.List,
+        };
+    }
+
+    private static string[] GetFilters(IReadOnlyList<FilePickerFileType> fileTypes)
+    {
+        return [.. fileTypes.SelectMany(fileType => fileType.Patterns)];
+    }
+
+    public async Task<string> GetPathAsync()
+    {
+        var folderPicker = new FolderPicker
+        {
+            ViewMode = PickerViewMode.Thumbnail
+        };
+        folderPicker.FileTypeFilter.Add("*");
+
+        InitializePicker(folderPicker);
+
+        StorageFolder folder = await folderPicker.PickSingleFolderAsync();
+
+        return folder.Path;
+    }
+
+    private void InitializePicker(object target)
+    {
+        var hWnd = WinRT.Interop.WindowNative.GetWindowHandle(mainWindow);
+        WinRT.Interop.InitializeWithWindow.Initialize(target, hWnd);
+    }
+}