Add a simple close button ("X") to windows.

Clicking the button generates a WindowCloseRequest event which the client app
then has to deal with. The default behavior for GWindow is to close() itself.

I also added a flag, GWindow::should_exit_event_loop_on_close() which does
what it sounds like it does.

This patch exposed some bugs in GWindow and GWidget teardown.

LibGUI/GWindow.cpp

@@ -35,11 +35,17 @@ GWindow::GWindow(GObject* parent)
 
 GWindow::~GWindow()
 {
+    if (m_main_widget)
+        delete m_main_widget;
     hide();
 }
 
 void GWindow::close()
 {
+    // FIXME: If we exit the event loop, we're never gonna deal with the delete_later request!
+    //        This will become relevant once we support nested event loops.
+    if (should_exit_app_on_close())
+        GEventLoop::main().exit(0);
     delete_later();
 }
 
@@ -160,6 +166,11 @@ void GWindow::event(GEvent& event)
         return;
     }
 
+    if (event.type() == GEvent::WindowCloseRequest) {
+        close();
+        return;
+    }
+
     GObject::event(event);
 }