BrowserSettings: Create a BrowserSettings application :^)

Browser has a bunch of settings, but most are non-trivial to add here.
So far, these are implemented:

- Homepage URL
- Whether to close download windows when they complete

The others will be added in subsequent commits.

Userland/Applications/BrowserSettings/BrowserSettingsWidget.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGUI/CheckBox.h>
+#include <LibGUI/SettingsWindow.h>
+#include <LibGUI/TextBox.h>
+
+class BrowserSettingsWidget final : public GUI::SettingsWindow::Tab {
+    C_OBJECT(BrowserSettingsWidget)
+public:
+    virtual ~BrowserSettingsWidget() override;
+
+    virtual void apply_settings() override;
+
+private:
+    BrowserSettingsWidget();
+
+    RefPtr<GUI::TextBox> m_homepage_url_textbox;
+    RefPtr<GUI::CheckBox> m_auto_close_download_windows_checkbox;
+};