LibGUI: Add SettingsWindow class

The FooSettings apps have quite a lot of boilerplate just around
creating a tabbed window with the same styling and the same row of
buttons along the bottom. So, let's extract that out into a class we can
reuse! :^)

You create a SettingsWindow instead of a regular Window, passing a title
and a flag to determine if a "Defaults" button is shown. Then call
add_tab() to add tabs to it. Tabs are widgets extending
SettingsWindow::Tab, which has methods for saving and resetting the
values.

Userland/Libraries/LibGUI/SettingsWindow.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2020, Idan Horowitz <idan.horowitz@serenityos.org>
+ * Copyright (c) 2021, the SerenityOS developers.
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGUI/Button.h>
+#include <LibGUI/TabWidget.h>
+#include <LibGUI/Window.h>
+
+namespace GUI {
+
+class SettingsWindow : public GUI::Window {
+    C_OBJECT(SettingsWindow)
+public:
+    class Tab : public GUI::Widget {
+    public:
+        virtual void apply_settings() = 0;
+        virtual void reset_default_values() {};
+    };
+
+    enum class ShowDefaultsButton {
+        Yes,
+        No,
+    };
+
+    virtual ~SettingsWindow() override;
+
+    template<class T, class... Args>
+    T& add_tab(StringView const& title, Args&&... args)
+    {
+        auto& t = m_tab_widget->add_tab<T>(title, forward<Args>(args)...);
+        m_tabs.append(t);
+        return t;
+    }
+
+private:
+    SettingsWindow(StringView title, ShowDefaultsButton = ShowDefaultsButton::No);
+
+    RefPtr<GUI::TabWidget> m_tab_widget;
+    NonnullRefPtrVector<Tab> m_tabs;
+
+    RefPtr<GUI::Button> m_ok_button;
+    RefPtr<GUI::Button> m_cancel_button;
+    RefPtr<GUI::Button> m_apply_button;
+    RefPtr<GUI::Button> m_reset_button;
+};
+
+}