Adds the ability to create key files (no entropy generator for now)

ModernKeePass/Controls/CompositeKeyUserControl.xaml.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using Windows.Storage.Pickers;
 using Windows.System;
 using Windows.UI.Xaml;
@@ -81,5 +82,20 @@ namespace ModernKeePass.Controls
             if (file == null) return;
             Model.KeyFile = file;
         }
+
+        private async void CreateKeyFileButton_Click(object sender, RoutedEventArgs e)
+        {
+            var savePicker = new FileSavePicker
+            {
+                SuggestedStartLocation = PickerLocationId.DocumentsLibrary,
+                SuggestedFileName = "Key"
+            };
+            savePicker.FileTypeChoices.Add("Key file", new List<string> { ".key" });
+
+            var file = await savePicker.PickSaveFileAsync();
+            if (file == null) return;
+            
+            Model.CreateKeyFile(file);
+        }
     }
 }