misc: chore: Multi/Single file/folder picker extensions (for convenience)

The result of these extensions is an empty Optional when the user hits Cancel on the shown file picker.

src/Ryujinx/Utilities/StorageProviderExtensions.cs

@@ -0,0 +1,47 @@
+using Avalonia.Platform.Storage;
+using Gommon;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Ryujinx.Ava.Utilities
+{
+    public static class StorageProviderExtensions
+    {
+        public static async Task<Optional<IStorageFolder>> OpenSingleFolderPickerAsync(this IStorageProvider storageProvider, FolderPickerOpenOptions openOptions = null) =>
+            await storageProvider.OpenFolderPickerAsync(FixOpenOptions(openOptions, false))
+                .Then(folders => folders.FindFirst());
+
+        public static async Task<Optional<IStorageFile>> OpenSingleFilePickerAsync(this IStorageProvider storageProvider, FilePickerOpenOptions openOptions = null) =>
+            await storageProvider.OpenFilePickerAsync(FixOpenOptions(openOptions, false))
+                .Then(files => files.FindFirst());
+        
+        public static async Task<Optional<IReadOnlyList<IStorageFolder>>> OpenMultiFolderPickerAsync(this IStorageProvider storageProvider, FolderPickerOpenOptions openOptions = null) =>
+            await storageProvider.OpenFolderPickerAsync(FixOpenOptions(openOptions, true))
+                .Then(folders => folders.Count > 0 ? Optional.Of(folders) : default);
+
+        public static async Task<Optional<IReadOnlyList<IStorageFile>>> OpenMultiFilePickerAsync(this IStorageProvider storageProvider, FilePickerOpenOptions openOptions = null) =>
+            await storageProvider.OpenFilePickerAsync(FixOpenOptions(openOptions, true))
+                .Then(files => files.Count > 0 ? Optional.Of(files) : default);
+        
+        private static FilePickerOpenOptions FixOpenOptions(this FilePickerOpenOptions openOptions, bool allowMultiple)
+        {
+            if (openOptions is null)
+                return new FilePickerOpenOptions { AllowMultiple = allowMultiple };
+
+            openOptions.AllowMultiple = allowMultiple;
+            
+            return openOptions;
+        }
+        
+        private static FolderPickerOpenOptions FixOpenOptions(this FolderPickerOpenOptions openOptions, bool allowMultiple)
+        {
+            if (openOptions is null)
+                return new FolderPickerOpenOptions { AllowMultiple = allowMultiple };
+
+            openOptions.AllowMultiple = allowMultiple;
+            
+            return openOptions;
+        }
+    }
+}