Centralize the setting of the pathbar's widgets

Centralize the setting of the pathbar's widgets

It used to be that every part of the file chooser's code would show/hide the widgets
near the pathbar as needed.  Now we have two central functions:

  path_bar_update()
  path_bar_set_mode()

These take care of all the widget shuffling;  setting the visibility of the
pathbar, info bar, and Create Folder button as appropriate; setting the contents
of the info bar, etc. - based on the current operation_mode and action.

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