WindowServer: Factor out window frame logic into a WSWindowFrame class.

The window frame is an object that contains a window, its title bar and
window border. This way WSWindowManager doesn't have to know about all the
different types of window borders, titlebar rects, etc.

Servers/WindowServer/WSWindowFrame.h

@@ -0,0 +1,22 @@
+#pragma once
+
+class Painter;
+class Rect;
+class WSMouseEvent;
+class WSWindow;
+
+class WSWindowFrame {
+public:
+    WSWindowFrame(WSWindow&);
+    ~WSWindowFrame();
+
+    Rect rect() const;
+    void paint(Painter&);
+    void on_mouse_event(const WSMouseEvent&);
+    void notify_window_rect_changed(const Rect& old_rect, const Rect& new_rect);
+
+private:
+    void handle_close_button_mouse_event(const WSMouseEvent&);
+
+    WSWindow& m_window;
+};