focusCaretTracker: Lower AT-SPI's timeout values

Have AT-SPI calls time out after 250ms, to mitigate the effect of a
deadlock when querying another application that is trying to query
gnome-shell.

https://bugzilla.gnome.org/show_bug.cgi?id=708387

js/ui/focusCaretTracker.js

@@ -33,6 +33,7 @@ const FocusCaretTracker = new Lang.Class({
 
     _init: function() {
         Atspi.init();
+        Atspi.set_timeout(250, 250);
         this._atspiListener = Atspi.EventListener.new(Lang.bind(this, this._onChanged));
     },