[KDirOperator] Use new "Show/Hide Hidden Files" standard shortcut

Summary:
CCBUG: 262551

Depends on D28802

Test Plan:
- Apply D28802 to KConfig
- Apply patch, compile, launch an app
- Open a file dialog
- Hit Ctrl+H or Alt+. and see that hidden files are shown or hidden as appropriate

Reviewers: dfaure, #frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D28803