Added PowerPoint templates to the Slide Assistant (#1055)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Tools/PowerPoint/IPictureSource.cs

@@ -0,0 +1,15 @@
+namespace AIStudio.Tools.PowerPoint;
+
+/// <summary>
+/// Hands out the pictures a slide deck refers to by number.
+/// </summary>
+internal interface IPictureSource
+{
+    /// <summary>
+    /// Loads a picture.
+    /// </summary>
+    /// <param name="number">The 1-based number the model used for the picture.</param>
+    /// <param name="token">The cancellation token.</param>
+    /// <returns>The picture, or null when there is no such picture or it cannot be read.</returns>
+    public Task<SlidePicture?> LoadAsync(int number, CancellationToken token);
+}