overview: Add focusSearch() method and export it over DBus

Some keyboard spot a dedicated search key, which gnome-settings-daemon
currently handles by spawning gnome-search-tool. It makes a lot of
sense to promote the Shell's integrated search feature instead, so
expose an appropriate DBus method g-s-d can use.

https://bugzilla.gnome.org/show_bug.cgi?id=700536

js/ui/overview.js

@@ -477,6 +477,11 @@ const Overview = new Lang.Class({
         this._animateVisible();
     },
 
+    focusSearch: function() {
+        this.show();
+        this._searchEntry.grab_key_focus();
+    },
+
     fadeInDesktop: function() {
             this._desktopFade.opacity = 0;
             this._desktopFade.show();