This is a pre-requisite for using a header bar.
GtkDialog:use-header-bar is a construct-only property, and has no
effect when set from the XML. Therefore, the only option is to turn
the widget into a template and set use-header-bar from C.
There should be no behavioral changes due to this patch.
Bump required GLib version for the G_DECLARE_FINAL_TYPE macro.
https://bugzilla.gnome.org/show_bug.cgi?id=757516