WindowServer+TaskBar: Add a taskbar window button popup menu.

This patch only hooks up the minimize and unminimize actions.

Applications/Taskbar/TaskbarButton.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <LibGUI/GButton.h>
+#include "WindowIdentifier.h"
+
+class TaskbarButton final : public GButton {
+public:
+    TaskbarButton(const WindowIdentifier&, GWidget* parent);
+    virtual ~TaskbarButton() override;
+
+private:
+    virtual void context_menu_event(GContextMenuEvent&) override;
+
+    GMenu& ensure_menu();
+
+    WindowIdentifier m_identifier;
+    OwnPtr<GMenu> m_menu;
+};