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
Instead of using the secondary slot for both clear and search. This
Makes it possible to use the search icon for actions regardless of
whether text has been entered, makes it possible to use the primary
icon to indicate search status, allows us to indicate the purpose
of the entry even if text has been already entered.
https://bugzilla.gnome.org/show_bug.cgi?id=694990
