trash-monitor: Rate limit updates

Trash monitor queries info from gvfsd-trash after each file monitor change which can be problematic when too many changes happen in a short time. Let's rate limit the number of queries...

I am going to propose the same patch for gtk+ trash monitor also...

This is part of my effort to reduce CPU load during mass changes of mounts, see also: https://bugzilla.gnome.org/show_bug.cgi?id=792331