Deprecate KFileWidget::toolBar

Summary: It leaks KToolBar from KXmlGui into the public API. I did not find any code using it

Test Plan: builds

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12206

Differential Revision: https://phabricator.kde.org/D25533