New GTK+ widget, GtkSearchEntry

Heya,

The widget is in GTK+ master. Could we have some recommendations drawn
up for more complicated uses, such as:
- Evolution and Rhythmbox' use of popups on the left-handside to select
the sources to filter (or go for the gnome-documents style drop-down)
- Whether we should recommend "search" buttons for non-instant searches,
or use of the activate button signal (pressing Enter on keyboard or
on-screen keyboard)

If we're going with gnome-documents drop-down, I'm currently writing a
widget for Totem to handle the drop-down selection, and uses
GdTaggedEntry to show the filters used.

Cheers