PixelPaint: Add a Selection class (ImageEditor has a Selection)

This will represent a complex, region-based selection in the future.
For now though, it's just a simple rectangle. :^)

Userland/Applications/PixelPaint/Selection.cpp

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Selection.h"
+#include "ImageEditor.h"
+#include <LibGfx/Painter.h>
+
+namespace PixelPaint {
+
+void Selection::paint(Gfx::Painter& painter, ImageEditor const& editor)
+{
+    painter.draw_rect(editor.image_rect_to_editor_rect(m_rect).to_type<int>(), Color::Magenta);
+}
+
+}