Move construction of ExpectedSignal where it belongs

Summary:
Deduplicate ExpectedSignal creation code by moving it where it
belongs, into proper constructors.

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D16555