WindowServer: Add a WSButton class and make the window close buttons use it.

Servers/WindowServer/WSButton.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <SharedGraphics/Rect.h>
+#include <AK/Function.h>
+#include <AK/Retained.h>
+
+class CharacterBitmap;
+class Painter;
+class WSMouseEvent;
+
+class WSButton {
+public:
+    WSButton(Retained<CharacterBitmap>&&, Function<void()>&& on_click_handler);
+    ~WSButton();
+
+    Rect rect() const { return m_rect; }
+    void set_rect(const Rect& rect) { m_rect = rect; }
+
+    void paint(Painter&);
+
+    void on_mouse_event(const WSMouseEvent&);
+
+    Function<void()> on_click;
+
+    bool is_visible() const { return m_visible; }
+
+private:
+    Rect m_rect;
+    Retained<CharacterBitmap> m_bitmap;
+    bool m_pressed { false };
+    bool m_visible { true };
+};