Add ::next-match, ::previous-match and ::stop-search keybinding
signals that are bound to Ctrl-g, Ctrl-Shift-g and Escape. Also
add a gtk_search_entry_handle_event() function to handle key events.
We add a GtkSearchEntry::search-changed signal which gets emitted
with a 150 millisecond delay. The ::change signal goes back to its
expected semantics.
https://bugzilla.gnome.org/show_bug.cgi?id=700229
