tracker-miner-fs: By default, ignore directories with '.trackerignore' inside

This fix updates the 'default' for the GSettings schema, to reset your config
(which previously had 'backup.metadata'), use:

  gsettings reset org.freedesktop.Tracker.Miner.Files
ignored-directories-with-content

Related to: https://bugzilla.gnome.org/show_bug.cgi?id=735317