New public bookmark_agent_update_from_bookmark_file() Main-menu uses two BookmarkAgent objects that monitor the same ~/.recently-used.xbel. As a result, that file gets read and parsed *twice* on every update.

With this, we start a new strategy to update BookmarkAgents.  They get
updated explicitly by the caller, not implicitly with a GnomeVFS file
monitor.  This also lets us have less files to monitor, and more
control for when expensive operations are performed.

Signed-off-by: Federico Mena Quintero <federico@gnu.org>

svn path=/trunk/; revision=417