Merge bgo593793-filechooser-recent-folders-master branch.

The file chooser now starts up in recent-files mode, or in recent-folders mode,
if the caller didn't set a current_folder on it.  Setting a current_folder is
now discouraged, so that the file chooser can have a chance of presenting
useful locations to the user.

See https://live.gnome.org/DocumentCentricGnome/Help%20the%20user%20choose%20a%20place%20to%20put%20a%20new%20file
for the rationale behind these changes.