Use libadwaita for extension preferences

This is the counter-part of !1997 (merged): When we adopt libadwaita for the extensions app, the actual dialogs should use libadwaita as well to make sure we use a consistent stylesheet (and dark-mode support).

This also replaces !1868 (closed) by providing official access to the dialog to extension authors.

The former bit should be uncontroversial, the latter could use some discussion:

• I'm not too enthusiastic about the fillPrefsWindow name

• AdwPreferencesWindow provides a standardized and convenient way for preference dialogs (good), but also limits what extensions can do (may be controversial)

Fixes #4248 (closed)