Add a convenient and race-free method of watching local files from the
GLib worker thread.

Without this, the race-free way to create a monitor that dispatches
events to the worker thread looked something like this:

 - dispatch an idle to the worker thread
 - from the idle, create the monitor and connect signals
 - from the original thread, wait (on a cond?) until the worker thread
   has finished setting up the monitor
 - read the file that you were monitoring

which is just ridiculously complicated...

To use the new API:

  monitor = g_local_file_monitor_new_in_worker ("/path/to/some/file",
                                                G_FILE_MONITOR_NONE,
                                                &error);
  g_assert_no_error (error);

  g_signal_connect (monitor, "changed", G_CALLBACK (callback), NULL);

  g_local_file_monitor_start (monitor);

'callback' will run from the GLib worker thread.

This is the reason that the start() call was introduced ...