Browser: Move main browser window logic into a BrowserWindow class

Having so much the logic and lambdas in main() was getting unwieldy.
Moving it into a class simplifies this, and also opens up a path
towards supporting "Open in New Window" :^)

Userland/Applications/Browser/BrowserWindow.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "BookmarksBarWidget.h"
+#include "WindowActions.h"
+#include <LibGUI/Window.h>
+
+namespace Browser {
+
+class CookieJar;
+class Tab;
+
+class BrowserWindow final : public GUI::Window {
+    C_OBJECT(BrowserWindow);
+
+public:
+    virtual ~BrowserWindow() override;
+
+    GUI::TabWidget& tab_widget();
+    void create_new_tab(URL, bool activate);
+
+private:
+    explicit BrowserWindow(CookieJar&, URL);
+
+    void set_window_title_for_tab(Tab const&);
+
+    CookieJar& m_cookie_jar;
+    WindowActions m_window_actions;
+    RefPtr<GUI::TabWidget> m_tab_widget;
+    RefPtr<BookmarksBarWidget> m_bookmarks_bar;
+};
+
+}