owncloud: Adjust the UI so that we can use a header bar in the dialog
owncloud: Adjust the UI so that we can use a header bar in the dialog

Using a header bar means that the dialog can emit both
GTK_RESPONSE_CANCEL (from the cancel button) and
GTK_RESPONSE_DELETE_EVENT (from the Esc key), so we should account for
that. This is also a good opportunity to stop using the action area.

Fixes: https://bugzilla.gnome.org/729638

halfline/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,Distrotech/gnome-online-accounts,halfline/gnome-online-accounts,Distrotech/gnome-online-accounts,felipeborges/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,felipeborges/gnome-online-accounts,felipeborges/gnome-online-accounts,Distrotech/gnome-online-accounts