Start in recently-used mode when no folder is set

Start in recently-used mode when no folder is set

In RELOAD_EMPTY mode, when no folder has been selected by the calling app, we now
start showing the recently-used list.  The rationale is as follows:

  - In Open mode, the user is likely to pick a file he has used recently.

  - In Save mode, the user is likely to want a destination folder which
    he has used recently.

For the Save case, where we want to present the user with recent folders instead
of recent files, we will make the recent-list do so in subsequent commits.

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