Convert credentials page to use HdyPreferencesGroup and a list box.

* Convert credentials page to use `HdyPreferencesGroup` and a list box.
* Add widget for displaying a credential provider in a list box.
* Convert `PidginCredentialsPage` into `HdyPreferencesPage`.

Testing Done:
Opened Preferences, switch to Credentials page, and see/toggle selected provider.

Reviewed at https://reviews.imfreedom.org/r/644/