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.cpp

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "BrowserSettingsWidget.h"
+#include <Applications/BrowserSettings/BrowserSettingsWidgetGML.h>
+#include <LibConfig/Client.h>
+
+BrowserSettingsWidget::BrowserSettingsWidget()
+{
+    load_from_gml(browser_settings_widget_gml);
+
+    m_homepage_url_textbox = find_descendant_of_type_named<GUI::TextBox>("homepage_url_textbox");
+    m_homepage_url_textbox->set_text(Config::read_string("Browser", "Preferences", "Home", "about:blank"));
+
+    m_auto_close_download_windows_checkbox = find_descendant_of_type_named<GUI::CheckBox>("auto_close_download_windows_checkbox");
+    m_auto_close_download_windows_checkbox->set_checked(Config::read_bool("Browser", "Preferences", "CloseDownloadWidgetOnFinish", false), GUI::AllowCallback::No);
+}
+
+BrowserSettingsWidget::~BrowserSettingsWidget()
+{
+}
+
+void BrowserSettingsWidget::apply_settings()
+{
+    // TODO: Ensure that the URL is valid, as we do in the BrowserWindow's change-homepage dialog
+    Config::write_string("Browser", "Preferences", "Home", m_homepage_url_textbox->text());
+
+    Config::write_bool("Browser", "Preferences", "CloseDownloadWidgetOnFinish", m_auto_close_download_windows_checkbox->is_checked());
+}