Add function to suggest a unique filename

Summary: This is a copy of KIO::suggestName. It doesn't use anything from KIO in its implementation and this patch enables to use it without pulling in all the baggage that comes with KIO

Test Plan: Port the one use in KDE Connect to it

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22620