gdesktopappinfo: Cancel file monitor when resetting a DesktopFileDir
gdesktopappinfo: Cancel file monitor when resetting a DesktopFileDir

It’s not enough to unref the monitor, since the GLib worker thread might
still hold a reference to it.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1903

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib