In order to propagate errors that occur during UI setup, we have to move
all that logic out of widget/window subclass constructors. This is a
first attempt at doing that, for GUI::SettingsWindow.
Some settings tabs, like the ones on the upcoming terminal settings,
need to know when the cancel button is pressed to clean up things like
temporary live updates. Therefore, the SettingsWindow::Tab now features
a cancel_settings callback which does not need to be implemented.
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.
