Ladybird: Trigger browser to quit when the main window is closed

This patch adds an event handler to the main window which allows it to
respond to a user closing the window. This event is then passed on to
the LibCore event loop, which allows the application quit itself.
Previously the application would hang, only running in the background,
until killed by an external force.

Ladybird/BrowserWindow.cpp

@@ -1,8 +1,10 @@
 #include "BrowserWindow.h"
 #include "WebView.h"
+#include <LibCore/EventLoop.h>
 #include <QStatusBar>
 
-BrowserWindow::BrowserWindow()
+BrowserWindow::BrowserWindow(Core::EventLoop& event_loop)
+    : m_event_loop(event_loop)
 {
     m_toolbar = new QToolBar;
     m_location_edit = new QLineEdit;
@@ -39,3 +41,13 @@ void BrowserWindow::page_favicon_changed(QIcon icon)
 {
     setWindowIcon(icon);
 }
+
+void BrowserWindow::closeEvent(QCloseEvent* event)
+{
+    QWidget::closeEvent(event);
+
+    // FIXME: Ladybird only supports one window at the moment. When we support
+    //        multiple windows, we'll only want to fire off the quit event when
+    //        all of the browser windows have closed.
+    m_event_loop.quit(0);
+}