Ticket #2101: allow skip directory scanning before file operations.

  * (compute_dir_size_create_ui): add "Skip" button to the "Directory
  scanning" progress dialog.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Handle "Skip" button in directory scanning status dialog.

Don't show total size, number and time values of file operations
if directory scanning was skipped before.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Get rid of g_return_* macros in file operation routines

...since they produce warnings about non-created UI and clutter screen
up.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
While scanning directories, print directory counter and size

...in addition to directory name.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>