smb: support more advanced hidden file flag

Summary:
smb attaches different meaning to the +x flags of files to represent DOS
style flags. read them and hide hidden files. we currently have no
KIO backing for the others but for documentation purpose I've mentioned
them in detail so we can extend this in the future without having
to descend into samba docs.

FIXED-IN: 19.12.3
BUG: 307353

Test Plan:
- set jpgs hidden as described in bug report
- they are now hidden in dolphin

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D27295