KSettings::Dialog: avoid duplicate entries due cascading $XDG_DATA_DIRS

Summary:
I had every toplevel entry doubled in kontact's configuration dialog
due to $XDG_DATA_DIRS=<myPrefix>/share:/usr/share
which led to parsing both
<myPrefix>/share/kontact/ksettingsdialog/knotes.setdlg
and
/usr/share/kontact/ksettingsdialog/knotes.setdlg

Reviewers: apol, broulik, davidedmundson, kossebau, svuorela

Reviewed By: svuorela

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28760