[KPropertiesDialog] Fix QLayout warning

Summary:
If a widget, QFrame in this case, already has a QLayout, adding/parenting
another layout to it shows a warning:

QLayout: Attempting to add QLayout "" to QFrame "", which already has a layout"

this happens when showing the properties of a symlink to a dir.

Reviewers: Frameworks, dfaure, meven

Reviewed By: meven

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28723