Emit the "changed" signal after 150 msecs, so that searching
through big lists, or doing online searches feels more responsive.
This is something already done in various applications to make
search-as-you type more responsive (gnome-shell, gnome-documents,
gnome-control-center, etc.). The 150 msecs is the value currently
used by gnome-shell, so keep it (invisibly) consistent.
https://bugzilla.gnome.org/show_bug.cgi?id=700229