Allow interaction when unmounting mounts

Allow interaction when unmounting mounts

For details, see bug 587482. The new api:

 - Provide new _with_operation() variants of all unmount and eject methods

 - Add GMountOperation::show-processes signal
   - this can be used to show processes blocking an unmount operation

 - Deprecate all unmount and eject methods

 - Add g_drive_can_start_degraded() method
   - this is to avoid auto-starting degraded drives

 - Make g_drive_stop() resp. g_file_stop_mountable() take a GMountOperation
   - these ops were recently added and not yet public API so it's fine
     to change how they work

 - Provide a way to poll mountable files, e.g. g_file_poll_mountable()

 - Add some missing file attributes for mountable files
  - G_FILE_ATTRIBUTE_MOUNTABLE_UNIX_DEVICE_FILE
    - needed for the GDU Nautilus extensions to format a volume
  - G_FILE_ATTRIBUTE_MOUNTABLE_CAN_START_DEGRADED:
    - mimics g_drive_can_start_degraded()
  - G_FILE_ATTRIBUTE_MOUNTABLE_CAN_POLL:
    - mimics g_drive_can_poll_for_media()
  - G_FILE_ATTRIBUTE_MOUNTABLE_IS_MEDIA_CHECK_AUTOMATIC
    - mimics g_drive_is_media_check_automatic()