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/CMakeLists.txt

@@ -84,6 +84,7 @@ set(SOURCES
     ScrollableContainerWidget.cpp
     Scrollbar.cpp
     SeparatorWidget.cpp
+    SettingsWindow.cpp
     Shortcut.cpp
     Slider.cpp
     SortingProxyModel.cpp