[chore:] implemented importer call with FilePicker

MainWindow.axaml.cs

@@ -1,7 +1,9 @@
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using Avalonia.Controls;
 using Avalonia.Interactivity;
+using Avalonia.Platform.Storage;
 
 namespace spplus;
 
@@ -71,9 +73,40 @@ public partial class MainWindow : Window
         w.Show();
     }
 
-    private void BtnImport_OnClick(object? sender, RoutedEventArgs e)
+    private async void BtnImport_OnClick(object? sender, RoutedEventArgs e)
     {
-        // CSV Import
+        var topLevel = GetTopLevel(this);
+        var file = await topLevel!.StorageProvider.OpenFilePickerAsync(new FilePickerOpenOptions
+        {
+            Title = "CSV-Datei auswählen",
+            AllowMultiple = false,
+            FileTypeFilter = new[]
+            {
+                new FilePickerFileType(".csv-Datei")
+                {
+                    Patterns = new[] { "*.csv" }
+                }
+            }
+        });
+
+        if (file == null) return;
+
+        var imported_students = import.ImportStudentsFromFile(file[0].Path.LocalPath.ToString());
+        foreach (var s in imported_students)
+        {
+            Settings.Instance.Students.Add(s);
+        }
+        RefreshImportedStudentList();
+
+    }
+
+    private void RefreshImportedStudentList()
+    {
+        LbStudentsImported.Items.Clear();
+        foreach (var s in Settings.Instance.Students)
+        {
+            LbStudentsImported.Items.Add(s);
+        }
     }
 
     private void BtnCraftCourses_OnClick(object? sender, RoutedEventArgs e)