PixelPaint: Start working on a custom layer list widget

Instead of using a TableView to show the layer stack, let's build a new
custom widget for this purpose and give it some neat features!

This patch also introduces an ImageClient interface for Image to notify
interested parties about things happening. The LayerListWidget is the
first ImageClient and listens for "layer added" and "layer removed"
notifications. :^)

Applications/PixelPaint/Image.cpp

@@ -67,6 +67,9 @@ void Image::add_layer(NonnullRefPtr<Layer> layer)
         ASSERT(&existing_layer != layer.ptr());
     }
     m_layers.append(move(layer));
+
+    for (auto* client : m_clients)
+        client->image_did_add_layer(m_layers.size() - 1);
 }
 
 GUI::Model& Image::layer_model()
@@ -126,6 +129,21 @@ void Image::remove_layer(Layer& layer)
     NonnullRefPtr<Layer> protector(layer);
     auto index = index_of(layer);
     m_layers.remove(index);
+
+    for (auto* client : m_clients)
+        client->image_did_remove_layer(index);
+}
+
+void Image::add_client(ImageClient& client)
+{
+    ASSERT(!m_clients.contains(&client));
+    m_clients.set(&client);
+}
+
+void Image::remove_client(ImageClient& client)
+{
+    ASSERT(m_clients.contains(&client));
+    m_clients.remove(&client);
 }
 
 }