Add "precision" to boxpointer alignment

Allows the boxpointer arrow to be at a fixed percentage of the bubble

https://bugzilla.gnome.org/show_bug.cgi?id=642031

js/ui/appDisplay.js

@@ -505,7 +505,7 @@ AppIconMenu.prototype = {
         if (St.Widget.get_default_direction() == St.TextDirection.RTL)
             side = St.Side.RIGHT;
 
-        PopupMenu.PopupMenu.prototype._init.call(this, source.actor, St.Align.MIDDLE, side, 0);
+        PopupMenu.PopupMenu.prototype._init.call(this, source.actor, 0.5, side, 0);
 
         this._source = source;