To make this work, we need to move the setting up of priv->builder
to the instance init, so that it is available when construct properties
are set; the other setup needs to remain in the constructor, since
it relies on construct properties.
https://bugzilla.gnome.org/show_bug.cgi?id=662489
Use a constructor so that the "shell" property is already set.
This allows use to block accels from working when capturing a key,
as well as parent dialogues properly for the toplevel.
