WindowServer: Add a window switcher.

This needs some work on the window ordering and things like that, but it
works quite nicely as a starting point.

The keyboard shortcut is Logo+Tab. :^)

WindowServer/WSWindowSwitcher.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <SharedGraphics/Rect.h>
+#include <AK/Vector.h>
+#include <AK/WeakPtr.h>
+
+class Painter;
+class WSKeyEvent;
+class WSWindow;
+
+class WSWindowSwitcher {
+public:
+    WSWindowSwitcher();
+    ~WSWindowSwitcher();
+
+    bool is_visible() const { return m_visible; }
+    void set_visible(bool);
+
+    void show() { set_visible(true); }
+    void hide() { set_visible(false); }
+
+    void on_key_event(const WSKeyEvent&);
+    void invalidate();
+
+    void draw(Painter&);
+
+    int item_height() { return 20; }
+    int padding() { return 8; }
+
+    WSWindow* selected_window();
+
+private:
+
+    Rect m_rect;
+    bool m_visible { false };
+    Vector<WeakPtr<WSWindow>> m_windows;
+    int m_selected_index { 0 };
+};