Let extensions access the extensionPrefs HeaderBar

[Felipe Borges <felipeborges gnome org>]

Hi,

I've recently reported a bug[0] about my desire of having a more
complex preferences dialog for my extension (powered by GtkStacks,
stack switchers et al just like modern gnome apps).

I understand that letting the extension devs handle all the extension
preferences dialog could be somehow 'too permissive', causing design
discrepancies. But still, I think that letting extension devs access
the extensionPrefs dialog headerbar could result in way better
designed dialogs.

If there's a proper way of achieving that without this patch, please
let me know. :)

Best,
Felipe.

[0] https://bugzilla.gnome.org/show_bug.cgi?id=743380