LibGUI: Add Window::on_close hook

This will be invoked when the window is closed, which is helpful if you
want to remove the window from some kind of owner after it's closed.

Libraries/LibGUI/Window.cpp

@@ -94,6 +94,8 @@ Window::~Window()
 void Window::close()
 {
     hide();
+    if (on_close)
+        on_close();
 }
 
 void Window::move_to_front()

Libraries/LibGUI/Window.h

@@ -84,6 +84,7 @@ public:
         Close,
     };
 
+    Function<void()> on_close;
     Function<CloseRequestDecision()> on_close_request;
     Function<void(bool is_active_input)> on_active_input_change;
     Function<void(const bool is_active)> on_activity_change;