Extend ctkDirectoryButton to optionally filter out readOnly folders.

For details see comment associated with acceptMode property.

See also CTK issue #365

Co-authored-by: Jean-Christophe Fillion-Robin <JChris.FillionR@kitware.com>
Merge branch '365-filter-out-read-only-directory'

* 365-filter-out-read-only-directory:
  Extend ctkDirectoryButton to optionally filter out readOnly folders.