There are two ways GTK can add a headerbar to a dialog:

 - the dialog is constructed with the :use-header-bar property

 - all windows should use client-side decorations

In the first case, the headerbar is added by GtkDialog with no
dedicated style class, and in the latter by GtkWindow with the
"default-decoration" style.

As a result, dialogs with plain titlebars can end up with clearly
distinct and inconsistent styles.

To address this, allow headerbars to track whether they should use
the "default-decoration" style and enable it for dialogs.

!836