Make GtkFileChoser remember the last directory opened

Make GtkFileChoser remember the last directory opened

Introduces a 'last-folder-uri' GSettings key, where we remember the last-opened
folder from the previous instance of the file chooser.

The idea is that this works globally, across all applications, so it will be
easy to do things like

  1. Save an attachment from a mail (or some other file)
  2. Open another program
  3. Do File/Open and automatically get sent to the folder where (1) happened.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>
https://bugzilla.gnome.org/show_bug.cgi?id=644426