Taskbar: More bringup work. We now see a basic window list.

Applications/Taskbar/WindowList.h

@@ -0,0 +1,80 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <AK/HashMap.h>
+#include <AK/Traits.h>
+#include <SharedGraphics/Rect.h>
+#include <LibGUI/GButton.h>
+
+class WindowIdentifier {
+public:
+    WindowIdentifier(int client_id, int window_id)
+        : m_client_id(client_id)
+        , m_window_id(window_id)
+    {
+    }
+
+    int client_id() const { return m_client_id; }
+    int window_id() const { return m_window_id; }
+
+    bool operator==(const WindowIdentifier& other) const
+    {
+        return m_client_id == other.m_client_id && m_window_id == other.m_window_id;
+    }
+private:
+    int m_client_id { -1 };
+    int m_window_id { -1 };
+};
+
+namespace AK {
+template<>
+struct Traits<WindowIdentifier> {
+    static unsigned hash(const WindowIdentifier& w) { return pair_int_hash(w.client_id(), w.window_id()); }
+    static void dump(const WindowIdentifier& w) { kprintf("WindowIdentifier(%d, %d)", w.client_id(), w.window_id()); }
+};
+}
+
+class Window {
+public:
+    explicit Window(const WindowIdentifier& identifier)
+        : m_identifier(identifier)
+    {
+    }
+
+    ~Window()
+    {
+        delete m_button;
+    }
+
+    WindowIdentifier identifier() const { return m_identifier; }
+
+    String title() const { return m_title; }
+    void set_title(const String& title) { m_title = title; }
+
+    Rect rect() const { return m_rect; }
+    void set_rect(const Rect& rect) { m_rect = rect; }
+
+    GButton* button() { return m_button; }
+    void set_button(GButton* button) { m_button = button; }
+
+private:
+    WindowIdentifier m_identifier;
+    String m_title;
+    Rect m_rect;
+    GButton* m_button { nullptr };
+};
+
+class WindowList {
+public:
+    template<typename Callback> void for_each_window(Callback callback)
+    {
+        for (auto& it : m_windows)
+            callback(*it.value);
+    }
+
+    Window& ensure_window(const WindowIdentifier&);
+    void remove_window(const WindowIdentifier&);
+
+private:
+    HashMap<WindowIdentifier, OwnPtr<Window>> m_windows;
+};