Fix ctkDirectoryButton file dialog for AcceptSave mode

In AcceptSave mode, the file dialog shall not accept non-writable folders.
2 mechanisms are in place:
 * a custom QSortFilterProxyModel for display information
 * a smart file dialog that does not allow non-writable folder to be selected
It still fails on Windows for some reasons.

Closes #365
Merge branch '365-reject-non-writable-folders'

* 365-reject-non-writable-folders:
  Fix ctkDirectoryButton file dialog for AcceptSave mode