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 FullPrefere...