Extensions must now export a class with a fillPreferencesWindow()
method in their prefs. That is less convenient for extensions
with simple preferences than the old buildPrefsWidget() hook, as
they must wrap their widget in page/group widgets.
Address this by adding a default fillPreferencesWindow() implementation
that calls a getPreferencesWidget() method and wraps it as necessary.
This is flexible enough to support different cases fairly conveniently,
from simple single-widget prefs over tweaking the window to complex
multi-page prefs:
```js
class SimplePreferences extends ExtensionPreferences {
getPreferencesWidget() {
return new SimplePrefsWidget();
}
}
class TinkerPreferences extends ExtensionPreferences {
getPreferencesWidget() {
return new SimplePrefsWidget();
}
fillPreferencesWindow(window) {
super.fillPreferencesWindow(window);
window.set_default_size(123, 456);
}
}
class FullPreferences extends ExtensionPreferences {
fillPreferencesWindow(window) {
const page1 = new GeneralPage();
window.add(page1);
const page2 = new FoobarPage();
window.add(page2);
}
}
```
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2838>
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