Move example from techbase to own repo

Summary: Add example based on https://techbase.kde.org/Development/Tutorials/KWallet so that it can be kept up-to-date. The techbase tutoriel can be archived and not maintained anymore.

Test Plan: It compiles

Reviewers: Frameworks, valir, dfaure

Reviewed By: dfaure

Subscribers: dfaure, aacid, elvisangelaccio, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14955