ImageViewer: Make arrow key shortcuts work

The user can now navigate to the previous and next image using the
left and right arrow keys respectively. These shortcuts were
previously not working.

Userland/Applications/ImageViewer/MainWidget.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2023, Tim Ledbetter <timledbetter@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "MainWidget.h"
+
+namespace ImageViewer {
+
+MainWidget::MainWidget()
+{
+    set_fill_with_background_color(true);
+    set_layout<GUI::VerticalBoxLayout>();
+    layout()->set_spacing(2);
+}
+
+void MainWidget::keydown_event(GUI::KeyEvent& event)
+{
+    event.ignore();
+}
+
+}