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.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGfx/Rect.h>
+
+namespace PixelPaint {
+
+class ImageEditor;
+
+// Coordinates are image-relative.
+class Selection {
+public:
+    Selection() { }
+
+    bool is_empty() const { return m_rect.is_empty(); }
+    void clear() { m_rect = {}; }
+    void set(Gfx::IntRect const& rect) { m_rect = rect; }
+
+    void paint(Gfx::Painter&, ImageEditor const&);
+
+private:
+    Gfx::IntRect m_rect;
+};
+
+}