Use GtkHeaderBar for the preferences dialog

Created by: bochecha

GNOME has published some new HIG for various kinds of dialogs.

In our case, we should follow these ones: https://wiki.gnome.org/Design/HIG/Dialogs#Presentation_Dialogs

Looking at it quickly, it seems that all we'd need to do is to use a GtkHeaderBar with a "X" (close) button, and that should be it.

However, GtkHeaderBar is only available in Gtk >= 3.10, so we'll need to keep the current code as a fallback somehow, or we'd lose compatibility with older platforms.